Terahertz dual-comb spectroscopy has become a powerful spectral measurement technology recently due to its high frequency resolution and high sensitivity. According to the sampling principles and methods of dual-comb spectroscopy in time domain and frequency domain, a terahertz spectroscopic system with tunable repetition frequency based on two femtosecond lasers is established in order to improve the detection performance of the spectroscopic system in this paper. By changing the repetition frequency of an optical frequency comb, the effect of repetition frequency difference on the terahertz dual-comb spectrum is analyzed systemically. The results show that the smaller the repetition frequency difference in the effective range, the higher the detected spectral quality. The detection performance of the terahertz dual-comb spectroscopy system is optimal when the repetition frequency difference is 10 Hz. These results provide a method to select the most suitable repetition frequency difference for terahertz dual-comb spectroscopy.