Abstract: The enhanced secondary electron emission caused by H₂O adsorption is the key factor leading to the abnormal discharge of vacuum microwave devices and equipment. In order to study the effect of H₂O adsorption on the secondary electron emission of metal surfaces, seven scattering types of electron-H₂O collisions were considered, and a Monte Carlo method was applied to simulate the scattering process between electrons and adsorbed molecules. Meanwhile, considering the effect of the change of work function on the probability of electron emission, we developed a secondary electron emission model of H₂O adsorption on the Cu surface. The final state of the electron are recorded, and the secondary electron yield (SEY) and secondary electron spectrum (SES) were analyzed according to the model. The results show that H₂O adsorption can increase the SEY by decreasing the surface work function and producing more ionized electrons. However, the SEY no longer increases until the adsorption thickness is greater than 100 nm due to the thick adsorption layer. The peak of SES is enhanced with the increase of adsorption thickness, indicating that H₂O plays a significant role in enhancing SEE by increasing the amount of low-energy electron emission. The proposed model provides a reliable analytical method for studying the secondary electron emission characteristics of complex surface states, and the relevant results provide theoretical guidance for the suppression of abnormal discharge in vacuum electrical equipment.