Abstract: A calculation method to calculate the surface shape error of rhombic prism according to parallelism demand of emergent beam is introduced. The relation of the surface shape error to f-number and relevant spherical radius was achieved according to the reducing analysis model ignoring the local f-number. Two kinds of rhombic prism model are established by ZEMAX. According to the simulation results from ZEMAX, the correctness and limitation of the former methods are proved and analyzed. The results are used to analyze the effect of the surface shape error of the rhombic prism on parallelism of the emergent beam. The assignation plan of the surface shape error for each working surface is presented. A conclusion that the effects of surface shape errors of the refractive and reflective surfaces on the deviation angle of the emergent beam are different with the increase of the incident beam diameter is achieved.