A new method to study wetetching process, infrared thermal image, is proposed. In wetetching process, the substrate of metal or semiconductor is dipped in chemical reagent, and the thermal energy and infrared emission are released because of the chemical energy. The signal of infrared emission is detected and sent to computer to be processed, the infrared thermal image is obtained, and process of wet etching can be analyzed by the signal. Theoretical analysis and experimental results show that the thermal diffusion can be monitored by the new infrared thermal image method directly, which is useful for monitoring and controlling wetetching. And it is also an expansion of infrared technology.