The method, by using photoconductive semiconductor switch to generate the offset voltage for terahertz time-domain spectrum system, was adopted to improve system’s performance. Based on the analysis of the terahertz wave’s generation mechanism by photoconductive semiconductor switch, the problems within the current terahertz time-domain spectrum system were fully examined, the solutions to solve the contradiction between the mechanic noise induced by chopper and the photoconductive antenna’s offset voltage, were discussed. Last, after testing the characteristic parameters of photoconductive semiconductor switch, a scenario which took advantage of high-voltage direct current source, photoconductive semiconductor switch, and nanosecond laser to generate 0 to 9 kV offset voltage without mechanic noise, was put forward to level up the amplitude of the offset voltage to improve terahertz time-domain spectrum system.