Abstract: To precisely measure the beam non-parallelism error of photoelectric autocollimators, a method based on a pentaprism has been proposed in this paper. By using a pentaprism, the measurement optical path can be optimized, thereby reducing the influence of surface shape irregularities of the mirror required in the detection process. The error of the non-parallel beam of the photoelectric autocollimator has been quantitatively analyzed, and different methods of beam parallelism measurement are compared experimentally. The results show that the difference between the pentaprism method and using a mirror with a curvature of less than 5×10⁻⁵ m⁻¹ is less than 0.1″, which is far superior to the traditional measurement method using a mirror with average flatness. Moreover, the repeatability of the pentaprism method can reach 0.1″. The pentaprism method reduces the system error and improves the measurement accuracy of the beam parallelism of the photoelectric autocollimator.