Photo-carrier transport has great influence on photoconductive antenna (PCA) terahertz electromagnetic wave. In this paper, involved with GaAs material′s two-valley effect, the Drude-Lorentz model is updated, and to some extent, the carrier transport conditions is obtained and compared with the simulation result of classical Diffusion-Drift model. Carrier density is coupled with the finite difference time domain (FDTD) method to simulate terahertz radiation′s time domain waveform which then be compared with THz-TDS′s measurement of the time-domain waveform. The results show that Drude-Lorentz model′s simulation time-domain waveform updated by two-valley effect is more similar with THz-TDS′s measurement waveform. When 14 μm and 34 μm aperture photoconductive antenna irradiated by titanium gem femtosecond laser, with its wavelength of 800 nm, pulse half width of 30 fs, pulse repetition frequency of 75 MHz, average power of 327 mW, the carrier′s transport changes caused by GaAs substrate′s two-valley play a key role in THz radiation.