Abstract: Conventional THz-TDS systems employ a stepped delay line for their optical delay line scan and utilize lock-in amplifier for acquisition of THz signal. In this approach, the speed of THz signal sampling is too slow to be introduced into some applications which require a high-speed performance. A terahertz time-domain spectroscopy (THz-TDS) based on fiber-coupled high-speed asynchronous optical sampling was proposed. An electronic system was used which included phase-locked loop (PLL) and the direct frequency synthesizer (DDS)to fix the frequency difference between the two femtosecond laser repetitions to achieve periodic liner increasing of phase delay and millisecond fast scan of the terahertz pulse. The results show that the system can shorten the scanning time of terahertz pulse to 2ms, and the actual sampling frequency can reach 1GHz. It effectively solves the problems of slow detection speed and poor portability in the past, and further expands the application range of terahertz spectrum measurement.