The deflection expressions,based on the vector refraction theorem,are derived from the light beam deflection through a pair of orthogonal prisms,and the main design parameters of the device are confirmed. The main error items are especially analyzed by the method of numerical simulation,and the total error and the actual precision index is also presented. The result shows the light beam,with the deflection precision of less than 0.8μrad,can be deflected 500μrad in both the horizontal and the vertical field angle,the deflection precision of which is mostly influenced by the random errors. The total error converted on the rotation angle of prism is 1 2.7 2 arcsec,which causes the deflection error of light beam 0.365μrad,greater than the reading resolution of system 0.0387μrad and less than the precision index of light beam deflection 0.8μrad,so the design is rational and feasible.