In order to achieve high repeatability and resolution of a 2-D ultra large stroke fast steering mirror(FSM) that integrates high-speed scanning and precise pointing functions, the 2-D segmental calibration was studied. The angle calibration system of 2-D ultra large strok FSM was constructed. The polynomial fitting method was optimized to obtain the segmental nonlinear fitting method, the calibration model was theoretically analyzed and experimentally tested. The results show that with the range of ±5°, the calibration error of the traditional polynomial fitting method is merely 1.7 mrad, while the fitting error of the optimized segmental nonlinear fitting method is less than 8.2 μrad, and the resolution of x and y axes can reach 4.6 μrad and 5.6 μrad respectively, and the repeatability of x and y axes is 19.4 μrad and 9.6 μrad respectively. This result plays an important role in the accurate control of 2-D ultra large stroke FSM.