1. A design method of forming a freeform surface of a freeform surface lens for an LED freeform surface illumination system comprising: S1, dividing a light emitting angle φ of a LED point light source into three regions, wherein the light emitting angle φ comprises a first light emitting angle φ N , a second light emitting angle φ S , a third light emitting angle φ T , φ satisfies 0≦φ≦φ T , the three regions are 0≦φ≦φ N , φ N ≦φ≦φ S , and φ S ≦φ≦φ T , each of the three regions forms a rectangular light spot on a light receiving surface, a length of the rectangular light spot is 2Lx, a width of the rectangular light spot is 2Ly, and the third light emitting angle φ T is a largest light emitting angle of the LED point light source; S2, obtaining a mapping relationship of each of the three regions on the light receiving surface; S3, performing a light redistribution design for the LED point light source and achieving a system of first order partial differential equations of a freeform surface, S4, obtaining a system of first order quasi linear differential equations of the freeform surface according to mapping relationships of the three regions on the light receiving surface and the system of first order partial differential equations of the freeform surface, obtaining a plurality of freeform surface data by solving the system of first order quasi linear differential equations; S5, obtaining the freeform surface by surface fitting the plurality of freeform surface data; and S6, arranging the freeform surface lens having the freeform surface obtained by the surface fitting the plurality of the freeform surface data.