According to the requirements of small angle measurement, with the auto collimation angle measurement technology, an auto collimation optical path mathematical model is built and the equation about the photoelectric position detector (PSD) image point coordinate Y and the flat mirror deflection angle α is deduced. PSD small angle measurement system is composed of preamplifier, A/D conversion circuit, sampling circuit and liquid crystal display. PSD position sensors output duplex angle information data, the ADC circuit completes A/D conversion, the single chip microcomputer completes data operation, in the end, the results of the position and the deflection angle are displayed on LCD screen. Simulation results show that the preamplifier circuit performance and single chip microcomputer sampling of 10-2 mrad level precision both meet the system requirements.