GaAs nanowires are treated with Ar plasma, and the effects of the bias power of plasma on the photoluminescence properties of the GaAs nanowires are studied by the photoluminescence test. The source and mechanism of each photoluminescence peak in photoluminescence spectra are studied at different temperatures and different excitation powers. The experimental results show that the free exciton emission of GaAs gradually disappears and the bound exciton emission decreases first and then increases with the increase of the power. When the power increases to 200 W, the donor-acceptor pair (DAP) emission appears. Comparing the photoluminescence spectra of different samples at 283 K, we obtain the structural changes of GaAs nanowires during plasma treatment. When the processing power is low, the Ar plasma eliminates the surface state and introduces a vacancy defect in GaAs; when the processing power is high, the crystal structure of GaAs is destroyed, and the DAP emission appears.