1. A method for determining BSIMSOI4 Direct Current (DC) model parameters, comprising the following steps: (1) providing a plurality of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) devices of a body leading-out structure and with different channel lengths L and different channel widths W; and providing a plurality of MOSFET devices of a floating structure and with different channel lengths L and different channel widths W, wherein 0.85Wmin≦W≦1 μm, 0.85L min ≦L≦1 μm, and W min and L min are minimum values determined by a process; (2) measuring electrical property data of all the MOSFET devices of a body leading-out structure and all the MOSFET devices of a floating structure in different temperatures by using a semiconductor parameter measurement instrument, wherein the measured electrical property curves of the MOSFET devices of a body leading-out structure comprise: Id-Vg-Vp, Id-Vd-Vg, Ip-Vd-Vg, Ig-Vg-Vd, Ig-Vp, Ip-Vg-Vd, Is-Vp, Id-Vp, Id-Vp-Vd and Ip-Vp-Vd property curves; and the measured electrical properties of the MOSFET devices of a floating structure comprise: Id-Vg-Vp, Id-Vd-Vg, and Ig-Vg-Vd property curves; (3) measuring heat resistance parameters Rth of all the MOSFET devices of a body leading-out structure and all the MOSFET devices of a floating structure; and obtaining, according to the electrical property data measured in step (2), electrical property curves without a self-heating effect of each MOSFET device of a body leading-out structure and each MOSFET device of a floating structure, wherein the electrical property curves without a self-heating effect of the MOSFET device of a body leading-out structure comprise: Id-Vg-Vp, Id-Vd-Vg, Ip-Vd-Vg, Ig-Vg-Vd, Ig-Vp, Ip-Vg-Vd, Is-Vp, Id-Vp, Id-Vp-Vd and Ip-Vp-Vd curves, and the electrical property curves without a self-heating effect of the MOSFET device of a floating structure comprise Id-Vg-Vp, Id-Vd-Vg, and Ig-Vg-Vd curves; (4) extracting, successively according to the following sub-steps, DC parameters of the MOSFET devices of a body leading-out structure and DC parameters of the MOSFET devices of a floating structure in a BSIMSOI4 model: (a) In dedicated software, according to actually applied test devices, process parameters tox, tsi, tbox, nch, and xj, layout parameters nf, nbc, pdbcp, and psbcp, and body contact resistance parametrs rbody, rbsh, and rhalo of each MOSFET device of a body leading-out structure and each MOSFET device of a floating structure are set; By using the dedicated software, in the bias voltage condition of Vg=0.5 Vgg, Vd=0.1 V, and Vp=0, with W=1 μm and L=L min , an Id ratio of the MOSFET device of a body leading-out structure to the MOSFET device of a floating structure in the normal temperature is calculated, the Id ratio is recorded as ratio, and a parameter dwc=(1-ratio)/nbc is obtained; wherein Id refers to drain current, nbc is a layout parameter of the device, and the dedicated software refers to MBP or BSIMProplus; (b) according to the electrical property data of the MOSFET device of a body leading-out structure with W=1 μm and L=1 μm, extracting parameters related to a body effect of the MOSFET device of a body leading-out structure in the BSIMSOI4 model, wherein the parameters refer vth0, k1, k2; (c) according to Is-Vp and Id-Vp curves of the MOSFET device of a body leading-out structure with W=1 μm and L=1 μm, extracting parameters related to a source body junction and a drain body junction of the MOSFET device of a body leading-out structure in the BSIMSOI4 model, wherein the parameters refer to ndiodes, ndioded, jdifs, jdifd, nrecfs, nrecfd, nrecrs, nrecrd, vrec 0 s, vrec 0 d, jrecs, jrecd, ntuns, ntund, vtun 0 s, vtun 0 d, jtuns, jtund; (d) according to Ip-Vp-Vd and Id-Vp-Vd curves of the MOSFET device of a body leading-out structure with W=1 μm and L=1 μm, extracting parameters related to a parasitic triode of the MOSFET device of a body leading-out structure in the BSIMSOI4 model, wherein the parameters refer to nbjt, lbjt 0 , jbjts, jbjtd, vabjt, aely, ahlis, ahlid; optimizing, by adjusting a parameter ln, fitting effects of Ip-Vp-Vd and Id-Vp-Vd curves of all the MOSFET devices of a body leading-out structure with W=1 μm, so as to determine the parameter ln of the MOSFET device of a body leading-out structure, wherein the parameter ln refers to the carrier diffusion lenth; (e) according to an Ig-Vp curve and an Ip-Vg-Vd curve of the MOSFET device of a body leading-out structure with W=1 μm and L=1 μm, extracting parameters related to a gate-to-body current of the MOSFET device of a body leading-out structure in the BSIMSOI4 model, wherein the parameters refer to αgb 1 , βgb 1 , vevb, vgb 1 , αgb 2 , βgb 2 , vecb, vgb 2 ; (f) according to an Ip-Vg-Vd curve of the MOSFET device of a body leading-out structure with W=1 μm and L=1 μm, extracting parameters related to a gate-induced drain leakage current of the MOSFET device of a body leading-out structure in the BSIMSOI4 model, wherein the parameters refer to agidl, agisl, bgidl, bgisl, cgidl, cgsil, egidl, egisl; (g) according to the electrical property data of the MOSFET devices of a body leading-out structure and of all sizes, extracting parameters related to a drain current Id and a gate current Ig of the MOSFET devices of a body leading-out structure in the BSIMSOI4 model; (h) according to an Ip-Vg-Vd curve and an Ip-Vd-Vg curve of the MOSFET device of a body leading-out structure with W=1 μm and L=1 μm, extracting parameters related to a collision ionization current of the MOSFET device of a body leading-out structure in the BSIMSOI4 model, wherein the parameters refer to α0, β0, β1, β2, fbjtii, vdsatii 0 , sii 0 , sii 1 , sii 2 , siid; optimizing, by adjusting parameters 1 ii and esatii, fitting effects of Ip-Vg-Vd curves and Ip-Vd-Vg curves of all the MOSFET devices of a body leading-out structure with W=1 μm, so as to determine the parameters 1 ii and esatii of the MOSFET device of a body leading-out structure, wherein the parameter 1 ii refers to the collision ionization length coefficient, and the parameter esatii refers to the collision ionization saturation electric field; (i) extracting temperature-related parameters related to a threshold voltage, mobility, a series resistance, saturation velocity, a junction current, a parasitic triode, and a collision ionization current of the MOSFET device of a body leading-out structure in the BSIMSOI4 model, so as to obtain all DC parameters of the MOSFET device of a body leading-out structure in the BSIMSOI4 model; and (j) first applying all the extracted DC parameters of the MOSFET device of a body leading-out structure to the MOSFET device model of a floating structure, then setting nbc, pdbcp, and psbcp to zero, keeping all of parameters related to a body bias voltage and parameters related to the gate-induced drain leakage current, the gate-to-body current, the junction current, and the parasitic triode unchanged, finely adjusting other DC parameters of the MOSFET device of a floating structure in the BSIMSOI4 model, and optimizing fitting effects of electrical property curves without a self-heating effect of the MOSFET device of a floating structure, so as to determine all DC parameters of the MOSFET device of a floating structure in the BSIMSOI4 model, wherein the parameters related to a body bias voltage refer to k 1 , k 2 , dvt 2 , k 3 b, cdscb, keta, uc, prwb, pdiblcb, cgidl, cgisl.