The physical mechanism of two-photon response(TPR)in semi-insulating CaAs is studied.The measured photocurrent generated from the fabricated hemispherical GaAs sample responding to 1.3μm continuous wave laser shows a quadratic dependence on the coupled optical power and no saturation with the bias.The angular dependence of the photocurrent on the azimuth is in agreement with the anisotropy of double-frequency absorption(DFA)in GaAs single crystals.These results demonstrate DFA is the dominant mechanism of TPR in GaAs.