Abstract: Because of the large thickness and low thermal conductivity of GaAs substrate in external-cavity surface-emitting laser, the heat diffusion of active region is seriously impeded and the laser output power is difficult to improve. In order to remove the GaAs substrate, sulfate acid corrosion was used. The corrosion effects were compared with different concentration etching liquid at different temperature. Atomic force microscope photos were used to represent the roughness of etching surface. The results show that when the volume ratio of the etching liquid V(H2SO4)：V(H2O2)：V(H2O) is 1：5：10 and the temperature is 30℃, the moderate etching rate about 5.2μm/min and the 2.7nm roughness of etching surface is obtained. The good etching effect of GaAs substrate provides the basic guarantee for the next step of the substrate removal-selective etch of the stop-layer.