Under the different experimental conditions, THz waveform which was generated by the different GaAs photoconductive switch provided with diverse carrier lifetimes triggered by femo-second laser pulse are calculated. The results indicate the main course of the dipole characteristic of THz waveforms emitting from low-temperature grown GaAs is the lifetime of optical-generated carriers less than the generation time; To SI-GaAs photoconductive semiconductor switches with the lifetime of optical-generated carriers more than 100 ps, the dipole characteristic of THz waveforms is mainly because of intra-valley scatter and the space charge field screening on different experimental conditions (different biased electric field and different optical pulse energy).