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 0.85L min ≦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 properties of the MOSFET devices of a body leading-out structure comprise: Id-Vg-Vp, Id/Ip-Vd-Vg, Ig-Vg-Vd, Ig-Vp, Ip-Vg-Vd, Is/Id-Vp, and Id/Ip-Vp-Vd properties; and the measured electrical properties of the MOSFET devices of a floating structure comprise: Id-Vg-Vp, Id-Vd-Vg, and Ig-Vg-Vd properties; (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/Ip-Vd-Vg, Ig-Vg-Vd, Ig-Vp, Ip-Vg-Vd, Is/Id-Vp, and Id/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 the bias voltage condition of Vg=0.5 Vgg, Vd=0.1 V, and Vp=0 V, and with W=1 μm and L=L min , calculating an Id ratio of the MOSFET device of a body leading-out structure to the MOSFET device of a floating structure, recording the Id ratio as ratio, and extracting a parameter dwc=(1-ratio)/nbc, wherein nbc is a layout parameter of the device; (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; (c) according to an Is/Id-Vp curve 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; (d) according to an Ip/Id-Vp-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 parasitic triode of the MOSFET device of a body leading-out structure in the BSIMSOI4 model; optimizing, by adjusting a parameter ln, fitting effects of Ip/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; (e) according to an Ig-Vp curve and an Ip-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 gate-to-body current of the MOSFET device of a body leading-out structure in the BSIMSOI4 model; (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; (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; 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; (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.