In order to solve the design problem of freeform mirror in ultra-short-focus projection system, a method of freeform surface design for large field of view imaging optical system is proposed, which is called multi-field optimization iteration method. In this method, a reflection plane is used as the design starting surface. Based on the corresponding relationship between objects and images in multi-field, and according to the normal direction vectors of the reflector, the freeform surface is obtained by weighted iterative optimization. Based on the freeform surface obtained by this method, a refractive-reflective combined ultra-short-focus projection objective is designed, which can magnify the 0.65 inch digital micromirror device chip to 100 inch projection screen at the projection distance of 230 mm. The modulation transfer function of the objective system is better than 0.4 at 0.43 lp/mm and the maximum distortion is less than 1%. This method is simple and feasible, which can provide useful reference for the design of freeform surface in large field of view imaging system.