To control of a Fast Steering Mirror(FSM) stably and precisely in the circumstances of vehicles and ships, a high precision device for angle displacement measurement was designed to provide the mirror sway angle information as a feedback signal to realize the servo closed loop control. As traditional detectors for the FSM are disadvantageous to high-precision work in the mobile circumstance, a precision grating based on the Moire fringe counting measurement principle was designed by using the Chinese characters Tian splitting phase method. At the same time, the absolute zero point signal against jamming and temperature changing was designed through computer simulation and its contrast ratio was 0.25. Furthermore, in order to realize the miniature, the signal processing circuit was highly integrated. Experimental results indicate that the resolution of the measurement device is 0.15″and the angle precision is higher than 0.4″. This device satisfies the requirements of angle measurement precision in the mobile condition for the high-energy laser projecting systems.