The angle measurement error detection of photoelectric encoder is an essential work in the development and production process of encoder. At present, the detection process of manual detection device is complex, inefficient and time-consuming, so it is difficult to be used for mass production product detection. In order to make up for the shortcomings of the existing manual detection device, this paper takes the autocollimator polyhedral prism combination as the measurement reference, uses the stepper motor and STM32 to realize automation, and designs an automatic detection system for angle measurement error of absolute photoelectric encoder. The detection principle, software and hardware design of the system are described. The accuracy of the system is analyzed and calculated. The expanded uncertainty is 1.6″. After manual and automatic detection of the same encoder, the results are 6.3″ and 7.3″, respectively. The experimental results show that the automatic detection result of the detection system is accurate. Through this system, the detection process does not need manual operation, and the efficiency is high. In the batch detection of photoelectric encoder, the advantages of this system can be reflected; In addition, compared with the traditional manual detection, this system can reflect the real working process of the encoder when it rotates at low speed.