Abstract: The detection concept for large diameter beam parallelism with penta prism is introduced. The effect of penta prism angle and surface profile errors on parallelism detection was analyzed. It is concluded that the angle error has no impact on the detection results and the profile errors of the two refraction surfaces have little effect, but the profile errors of the two reflection surfaces have great influence on the detection. The methods for selecting penta prism and reducing the effect of surface profile error on detection are introduced. At the end, an example is given to illustrate the detection of the parallelism error of a single star simulator with penta prism.