Abstract: The projection angle of two B-edges of pentagonal prism has a great effect on the second optical parallelism; however, it cannot be measured during on line processing, which results in low qualification rate. By analyzing the impact of perpendicular error of the side face and angle error of 45 on the optical parallelism of pentagonal prism, this paper provides a new process for producing pentagonal prism, which modified the angle error of the last face to compensate the angle errors of three processed faces so as to release the strict requirements for process error of these surfaces. By using this process, qualification rate and productivity was raised, the complexity of the process was reduced, and the strict requirements for the machine and measurement equipments were released.