An infrared thermalimage method which is used to study the wet etching process of semiconductor is proposed. The substrate of semiconductor is dipped into a chemical bath,the infrared radiation signal of etching is detected by the infrared thermo-image instrument and sent to computer to process,the picture of infrared thermo-image can be obtained by this method. The wet-etching of GaAs in different chemical bathes is discussed. Theoretical analysis and experimental result show that the distinct difference of experiment can be monitored by the new infrared thermo-image method,and the relative relation of the three main factors-substrate,chemical baths and time of etching--can be indicated by the method directly.