Influence of the geometrical parameters of GaAs dipole photoconductive antenna(PCA) on the PCA′s radiation characteristics was studied. At first, the analytical solution of PCA′s current obtained by the Drude-Lorentz(DL) model was obtained as excitation, and the numerical finite-difference time-domain(FDTD) simulation was conducted. This had effectively solved the problem of analytical solution (where the relative intensity of THz radiation can be obtained by time derivative) which cannot simulate the influence of PCA structure parameters. Then, using the equivalent circuit model of dipole photoconductive antenna and the equivalent source resistance′s expression, both the PCA′s and the source′s impedances were obtained for the practical PCAs. And it is demonstrated that the matching efficiency is not high because of the very low source impedance. Finally, by considering both the FDTD simulation results and theoretical calculation results, it can be proved that the dipole antenna′s radiation efficiency will increase with the increase of its aspect ratio. This conclusion is found to be in accordance with the reported measured results. Thus the simulation results, the theoretical model and the calculation results are all verified.