1. A design method of LED freeform surface illumination system comprising: S 1 , dividing a light emitting angle φ of a LED point light source into three regions; wherein 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 2L x ; a width of the rectangular light spot is 2L y , and φ T is largest light emitting angle of the LED point light source; S 2 , obtaining a mapping relationship of each of the three regions on the light receiving surface; S 3 , 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, S 4 , 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; and S 5 , obtaining the freeform surface by surface fitting the plurality of freeform surface data.