Abstract: Simulation characteristics of photoconductive antenna(PCA) radiation are the important basis for design and manufacture of photoconductive antenna. The method introduced the relationship between Monte-Carlo empirical-formula and updated GaAs migration-rate versus electric-field strength. The photo-generated carrier-density of 34 m-apeture GaAs dipole photoconductive antenna was computed based on Drude-Lorentz model. Combining the photo-generated carrier-density with finite difference time domain(FDTD) method, time-domain, frequency-domain and three-dimensional radiation characteristics were simulated. The frequency spectrum of the terahertz radiation ranged about 1 THz, and its amplitude peaks were at roughly 0.8 THz, which consisted well with the data obtained from terahertz time-domain spectrum system(THz-TDS). The results show the feasibility of this method, which provides a reference for design and manufacture of the high-gain, wideband THz photoconductive antenna.