Long-distance and large-range beam translation is achieved in the optical scanning and photodetection systems by analyzing the accuracy of the beam translation system comprised of a double pentaprism and by obtaining the parallel accuracy of the system using the autocollimation method. The direction of the emergent light in the system is not parallel to that of the incident light because of the errors in the positions of two pentaprisms in an component relative to the theoretical position. The autocollimation method can be used to optically adjust the system to ensure that the direction of the emergent light is parallel to that of the incident light. The beam-translating component comprising a double pentaprism rotates around the incident optical axis by the rotating pair during operation; hence, the whole system will generate a rotational error around a certain mechanical axis, which will cause the coordinate transformation matrix of the double pentaprism system to no longer be the unit matrix and the direction of the emergent light to no longer be parallel to the direction of the incident light. Further, a mathematical model can be established by analyzing the azimuth error of the pentaprism and the nonlinear relation between the overall rotation error and the systematic error of the system. The ray-tracing simulation is used herein to verify the theoretical analysis accuracy. A solution is also proposed for reducing the systematic error, which controls the accuracy of the beam translation system comprising double pentaprisms to within 10″ and provides a good reference for the error analysis of the prism in case of the beam folding problem.