It was experimentally investigated that the laser spot size on photoconductive antenna(PCA) gap could have a great influence on terahertz(THz) generation. Moreover, the simulation on THz generation influenced by the laser power density was carried out, which was agreed well to the experiments. It is well demonstrated that the increase of laser power density on PCA gap can enhance THz generation. However, the intensity of THz radiation reached saturation while the laser power density exceeding a certain value. The laser power density increased further by focusing harder, but the THz radiation was getting weaker due to the smaller laser excited area on PCA gap.