Abnormal discharge of laser resonator is studied by using the NEL-A series high-power fast-flow axis CO2 laser. By chemical experiment, it is verified that the main corrosion of laser resonator is the basic carbonate copper. Based on physics and chemistry theory, the causes of basic carbonate copper and the micro-mechanism of abnormal discharge of laser resonator are analyzed. The testing results show that the basic carbonate copper is the main reason of abnormal discharge. The conclusion can be used as a reference to improve the design of laser resonator and gas supply unit. At the same time, it provides a new way for equipment maintenance on the spot.