An optimization design method combining surface optimization strategy and field-of-view optimization strategy was presented for expanding the field-of-views, which is used for the design of off-axis reflective optical system with freeform surface. Based on the coefficients of Zernike standard polynomials representing the wavefront error of the optical system, sequence of the aberration was determined. The optimization procedure was implemented in small field-of-views to achieve satisfied results, and then the field-of-view was expanded gradually, and the optimization steps were repeated. At last, the optical system structure parameters were got for the full field-of-views. With proper XY polynomial terms as well as optimization weight of different field-of-views selected and optimized, an F-12 off-axis three-mirror system was designed with 1 200 mm focal length and 30°×3° field-of-view, in which, only 15 XY polynomials terms were adopted for the freeform surface. The imaging quality of the system is close to the diffraction limit that meets application requirements. This method can also guide the design of optical systems with freeform surface and larger field-of-view.