Based on the density functional theory, the mechanism of residual gas adsorption on the surface of EBCMOS passivation layer is investigated and its electrical characteristics are analyzed. The results show that the adsorption of residual gas H2 in the cavity on different surfaces of the passivation layer (taking Al2O3 as an example), are physisorbed, with the largest adsorption distance, the lowest adsorption strength and the least charge transfer on the (001) surface. The compared results of the adsorption of other residual gas molecules such as CO, N2, CO2, H2O on the (001) surface show that all of them are physically adsorbed, and the (001) surface has a better inhibition of residual gas molecules compared to other surfaces, and the generation of passivation layer is more capable of improving the electron collection efficiency of the multiplication layer of EBCMOS device. Theoretical guidance is proposed for the development of long-life and stable EBCMOS device.