In order to control rotary angle of reflector accurately in the Fast-steering Mirror (FSM) system, a specialized grating sensor used for measuring the position of reflector to offer feedback information for servo controlling system was designed. On the basis of application requirements of grating sensor in the fast-steering mirror system, small slip track, pattern of scale gratings, pattern of index gratings, form of absolute zero code, moving index gratings and styles of measure probes were designed and chosen respectively on technology principle of grating sensor. Then, the grating sensor was tested after finely manufacturing, assembling and adjusting. The results show that measure length of the grating sensor is about ± 2.5 millimeters, its response frequency is about 1 000 Hz, its measure error is less than 2 microns, its measure differentiate is less than 0.06 microns and it can export absolute zero. Applied results of the grating sensor show that the sensor which has high precision, excellent resolution and powerful dependability can satisfy application requirements of the fast-steering mirror system.