According to the theory of semiconductor optoelectronics, the transient transport mechanism of light-injected non-equilibrium carriers (electron-hole pairs) was analyzed, the saturation effect of THz wave radiation power and radiation intensity in SI-GaAs photoconductive dipole antenna under different conditions was studied. The main reason is that, under the action of DC bias electric field, the space charge electric field shielding and radiation electric field shielding will appear in optical injected carriers, thus the radiation power and intensity of THz wave are contained. The two shielding effects are different for antennas with different electrode gaps, the shielding effect is small when the light spot is large under the triggering light energy is certain.