This paper studies the enhancement effect and mechanism of surface chemical sulfur-selenium passivation technology on the performance of gallium arsenide-based light-controlled terahertz modulation devices. Experiments show that sulfur-selenium passivation can effectively remove gallium arsenide surface oxides, reduce surface recombination centers, and improve the surface state of gallium arsenide. After passivation, the Photoluminescence(PL) intensity is significantly enhanced, which is 6 times that of the reference gallium arsenide substrate. At the same time, the passivation effect can significantly increase the lifetime of minority carriers in gallium arsenide, up to 2.2 ns. The surface passivation effect can significantly improve the modulation performance of the GaAs-based terahertz modulator. The measured modulation depth is 41% and the modulation rate is 88.81MHz under 3 mW laser power. The optically controlled terahertz modulator has high modulation depth and modulation rate at low power, and has huge application potential in the field of terahertz communication.