Abstract: In reflective light path,because of the errors in assembling,planar reflector would have three tiny dimensional angles stray.In this paper,according to the law of prism turning,due to the errors in planar reflector assembling,light axis departure from systemic axis and light turning are analyzed.Based on this,the precise formula is deduced.Meanwhile,the stray of spot of light on the target resulting from light axis departure is discussed.In order to modify axial deflection of planar reflector in assembling,an adjusting method is presented and verified through experiment.