Aiming at the problem of low accuracy of traditional external modulation ranging method based on pilot signal, a phase laser ranging method based on switching circuit bias control module is proposed. Firstly, the reasons for the low accuracy of the traditional ranging method are analyzed. Then, a bias control module based on switching circuit is designed, which changes the traditional way of inputting coarse scale signal and fine scale signal at the radio frequency end at the same time, and inputting coarse scale signal and fine scale signal at the radio frequency end of Mach-Zender modulator(MZM) to realize bias control and laser distance measurement simultaneously. The experimental results show that the repeatability of phase difference is ±0.150 4° and the accuracy is ±0.026 1 cm when 500 measurements are made at a distance of 80 cm.