The photoconductive terahertz(THz)radiation theory is analysed,and the radiative process of photoconduction is deduced based on InSb.The parameters of near and far radiation field can be obtained.The approach of Finite Difference Time Domain(FDTD)is used in the analysis to the airspace spread of THz waves,and the Terahertz spreading 3D effect graphs are obtained.The analysis results match the experiment data well which validate the correctness of the research.