The working stability of the optical fiber-coupled Terahertz time-domain spectral system is mainly limited to the interpolation stability during the delay scanning of the optical delay line. A new high-stability optical delay line scanning device is designed to solve this problem. Optical simulation tolerance analysis of the new optical delay line and the traditional optical delay line shows that when its internal the pyramide mirror and the collimator have the same degree of tilt and eccentricity conditions, the coupling efficiency fluctuation of the new optical delay line is much smaller than that of the traditional optical delay line. The experimental results of terahertz time-domain waveforms comparison show that the peak fluctuation of the terahertz time-domain signal obtained by the new optical delay line designed by this scheme is only 1/4 of that of the traditional optical delay line. A feasible scheme is provided to improve the stability of fiber-coupled terahertz time-domain spectroscopy system.