Abstract: The small dipole aperture antenna provides a normal way for THz radiation,for which the Drude-Lorentz theory provides method to calculate the terahertz waves from photoconductor.For sake of this Drude-Lorentz theory is not suitable for large-aperture photoconductive antenna,the calculation of terahertz radiation from biased photoconductive antenna(Drude-Lorentz theory) is reported,and a meliorated calculation method of carriers' density is put forward.Based on the new calculation model and simulation results,the effect of different factors on THz radiation is analyzed.The results show that the new calculation model is more suitable for a large aperture antenna.