Photoconductive antenna is one of the most useful radiation sources for terahertz generation and detection，which has broad applications in the terahertz imaging，spectral detection and many other areas. In this paper，a calculation method of THz photoconductive antenna 3-D radiation characteristics by using Finite-Difference Time-Domain is presented. By analyzing the radiation theory of photoconductive antenna，the effect of semiconductor drift current and diffusion current on the electric and magnetic field is described. According to the actual situations，the drift equation and continuous equation can be simplified. Finally，the iterative equations of current density, electric and magnetic field are obtained, and the computing process for the photoconductive antenna radiation characteristics is introduced.