The resonant frequency of dither mechanism of laser gyro varies with the temperature and other environmental factors, resulting in dither bias instability. The resonant frequency tracking technology of dither mechanism of laser gyro based on phase-locked loop is investigated. Based on the analysis of the transfer function of the dither mechanism and the phase-locked loop, the general relationship between the performance of the frequency tracking accuracy and the range of synchronization and the dither control parameters is worked out with the automatic control theory. The results show that the increase of the open-loop gain of phase-locked loop can reduce the steady-state error of frequency tracking and the synchronization range of phase-locked loop simultaneously. The low open-loop gain is adopted to initiate laser gyro, and then high-open loop gain is used to track frequency of dither mechanism. This strategy can guarantee the gyro resonance frequency being tracked automatically within 100 ms. The frequency tracking accuracy of the dither mechanism of a laser gyro machine based on phase-locked loop is better than 0.015 Hz with temperature ranging from -40 ℃ to 70 ℃.