Abstract: Ultra-fast terahertz time-domain spectroscopy(THz-TDS) system operates based on the principle of high-speed asynchronous optical sampling. The system uses two femtosecond oscillators whose repetition frequencies can vary around 1 GHz, and uses a high-bandwidth feedback circuit to control the repetition frequencies. There is a difference of Δf in the repetition frequency between two femtosecond oscillators. One femtosecond oscillator has a repetition frequency of 1G+Δf Hz, which is the pump pulse; the other has a repetition frequency of 1G Hz, which provides the time difference between the pump pulse and the probe pulse, and the time delay varies periodically with the scanning cycle. The period can be given by 1/Δf. This system abandons the mechanical delay line necessary for the traditional THz-TDS system and uses double-photon detector to generate trigger signal. When the frequency difference between the two femtosecond laser oscillators is 1 kHz, a THz spectrum can be detected in only 1 ms, and a THz signal with a dynamic range of 21 dB and a spectral resolution of 5 GHz can be obtained in 10.3 s. Hence it can be characterized by fast detection speed and high resolution. It has incomparable advantages over the traditional THz-TDS system in the application environment that requires rapid measurement.