Terahertz time-domain spectroscopy is a powerful tool for spectral research in terahertz band. But its detection speed and signal quality need to be further improved. An all fiber direct acquisition terahertz time domain spectrum system based on fast optical delay line is proposed. The experimental results show that the scanning frequency of the system can reach 16Hz under 64ps time window, and the dynamic range of frequency domain is more than 85dB. Compared with the traditional phase-locked scanning system, not only the signal quality of single frame THz time-domain pulse is better, but also the detection time is shortened to 1/40 of the original, which provides a feasible solution for the practical application of THz time-domain spectrum system.