Gear pair gap is an important parameter in gear mechanical design and gear meshing.In order to improve the measurement precision of the gear pair gap and solve the problem that it is impossible to measure the gap of the gear pair which is equipped with a circular grating photoelectric code disk on both gears as an angle sensor due to the limitation of structure, an automatic measuring technique based on the photoelectric angle measurement module is proposed for measuring the gap of the gear pair.To collect the angles of the two gears, a micro-photoelectric angle measurement module is installed on the driving gear and a photoelectric encoder is mounted on the driven gear, and then data processing and software control are performed to realize the automatic measurement of the gear pair gap.The main error sources affecting the automatic detection system are given, and an error analysis is carried out.Finally, the automatic measurement results of the gap of bevel gear pair are given through experiments.