Focal plane’s misalignment is analyzed theoretically to match the requirement of high precision CMOS star sensor. System for assembly and calibrating measurement of CMOS star sensor is established. Defocusing distance and tilt of detector plane are acquired by on-axis and off-axis star sensor imaging test through collimator’s image plane moving. Some CMOS star sensor with 50 mm focal length, F/1.25, and 20° field of view is successfully assembled under direction of testing data. Defocusing distance is less than 0.01 mm, and tilt is less than 2′. Principal distance test and distortion calibration are done with measuring system. Principal distance is calculated to be 49.77 mm, and the error is 0.007 2 mm. Distortion of star sensor system is calibrated by third order polynomial fitting data, and the residual error caused by distortion can be reduced to 6.6″ when measuring single star.