Terahertz time domain spectroscopy system (THz-TDS) is the most commonly means of measuring terahertz time-domain spectroscopy. The time delay between the pump and probe laser is an important technology to realize THz time domain spectrum measurement. The translation platform with two mirrors and the mechanical structure is the popular means to adjust the optical path difference between the pump and probe laser to get the time delay of femtosecond pulse. Because of the limit of the mechanical structure and the phase-locked amplifier, this technique can’t scan spectrum fast. In order to obtain high quality signal, a long time will be taken to scan spectrum. So a more rapid and convenient time delay technology is required to Instead of the machine translation platform and accomplish the Rapid spectral measurement. Asynchronous optical sampling technique is a way to get the time delay by producing a very small frequency difference between the repetition frequency of two femtosecond lasers. The scanner time will be reduced, because of there is no waste of time, due to mechanical inertia, not only by using the asynchronous optical sampling method to replace the mechanical structure without the influence of vibration. It will greatly increase the degree of integration by using the fiber femtosecond laser and highly integrated circuit to realize optical asynchronous sampling.<p> </p>To solve the problem above, a terahertz time-domain spectroscopy system based on asynchronous sampling is designed in this thesis. The system is based of two femtosecond laser whose repetition frequency is 100MHz.In order to realize asynchronous sampling, the control circuit of the two lasers is the most important. This thesis focuses on the researching, designing and experiment of this circuit. Firstly, the circuit is designed overall. Then the selection of the key device and the designing of the circuit principle is done by myself. Secondly, the test of the circuit to phase locked the master and slave fiber femtosecond lasers has been done. As a result, the two lasers can phase locked on two repetition frequencies with a small frequency difference calculated by the circuit.