The photoelectric autocollimator has the characteristic of strong optical path dependence in the angle measurement process, so it is easy to cause unstable measurement results. In order to solve the above problem, this paper analyzes the influence of the partial occlusion of the optical path of the system on the measurement error, and establishes a mathematical model of the angular error to test the autocollimator. Combined with the analysis results, the geometric parameters of the splitter were optimized to reduce the dependence of the measurement results on the optical path, and the optical system of the autocollimator was further optimized to deal with the fluctuation of the measurement results caused by the low energy transmission efficiency after the narrowing of the slit. At the same time, the optical path structure of double-CCD sampling is proposed to reduce the data fluctuation caused by random errors or the false target caused by too few data. The results show that the designed autocollimator optical system has the advantages of good stability and simple structure. The modulation transfer function value of each field at 62.5 lp/mm is higher than 0.2, the maximum distortion value is less than 0.0052%, and the energy transmission is close to the diffraction limit. It provides reference for improving the measurement stability of the photoelectric autocollimator.