In this paper, a terahertz dual-comb ranging system is constructed based on a fully bias-maintaining erbium-doped fiber laser, phase-locked loop system and terahertz ranging optical path. In the experiment, the full bias-maintaining erbium-doped fiber laser had a repetition frequency of 79.261 MHz, and the repetition frequency was locked and the frequency was adjustable at 1.54 MHz by piezoelectric ceramic (PZT) and stepper motor (SM) dual-stage feedback control scheme. The peak-peak jitter and standard deviation of jitter were ±1.5 mHz and 0.4 mHz respectively. The repetition frequency difference of the dual-comb was set at 10 Hz, and the maximum jitter and standard deviation of repetition frequency difference within 10 min were calculated to be 3 mHz and 0.6 mHz. Furthermore, the asynchronous sampling dual-comb system was applied to the terahertz ranging, and the error of measuring the moving distance was 3 μm. The system has the advantages of high locking precision and strong stability, and is expected to be used in biological nondestructive testing and industrial precision machining.