In this paper，different methods are used to simulate the dipole characteristic of terahertz (THz) wave emitted from low-temperature grown GaAs (LT-GaAs) and Semi-insulting GaAs (SI-GaAs) photoconductive semiconductor switches. The results indicate that the main cause of the dipole characteristic of THz wave emitted from LT-GaAs is the lifetime of optical-generated carriers being shorted than the generation time. For SI-GaAs photoconductive semiconductor switches with lifetime of optical-generated carriers longer than 100ps，the dipole characteristic of THz waveforms is mainly caused by intra-valley scattering and space charge field screening under different experimental conditions (different bias field and different optical pulse energy).