The GaAs photoconductive switch is an important device in the measurement of the signal of ultra high-speed. In this paper, the transmission model of transient signals on GaAs photoconductive material is given, and the experimental system of near-field detection is set up. It is gotten that the full width half magnitude of signal is about 2ps. Based on the testing result of delay-time-modulation method, the relationship between signal amplitude and the form of connection mode is analyzed and discussed. It is shown that the coupling of capacity is important to acquire the transient signal.