According to the theory of semiconductor optoelectronics, the transient transport mechanism of light-injected non-equilibrium carriers (electron-hole pairs) is analyzed, The saturation effect of THz wave radiation power and radiation intensity in SI-GaAs photoconductive dipole antenna under different conditions is studied. The main reason is under the action of DC bias electric field, the space charge electric field shielding and radiation electric field shielding appear in optical injected carriers, 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 lase spot is large under the triggering light energy is certain.