The dual-comb system has its unique advantages and broad development prospects in the fields of high precision absolute distance measurement, 3-D imaging and spectral measurement. In this paper, an asynchronous locking technique based on phase-locked loop principle was proposed, which can precisely lock the repetition rate difference in dual-comb system. In the experiment, a dual-comb system with a repetition rate of about 155.711 MHz and 155.714 MHz was adopted. By controlling the driving voltage of PZTs through feedback, the length of the laser resonator was controlled to achieve the locking of the repetition rate difference, and the frequency counter was used to collect the data of the repetition rate difference after locking. Allan deviation and standard deviation were used as evaluation indexes of frequency stability. Finally, within the average time of 1 s, the Allan deviation of repetition rate difference was 1.8×10^-13, and the jittering standard deviation was 40.689 μHz. The results show that this technology can adjust the repetition rate difference flexibly, and has the advantages of high locking accuracy and strong disturbance rejection.