To investigate the issue of reduced electron multiplication factor in Complementary Metal Oxide Semiconductor(CMOS) image sensors with an aluminum oxide passivation layer structure after being bombarded with high current density electrons, this paper simulates the fabrication process of CMOS image sensors with an aluminum oxide passivation layer. An aluminum oxide passivation layer is prepared on the surface of P-type silicon with a crystal orientation of (100) and a doping concentration of 5×10¹⁸ cm^-3. The conditions for electron beam bombardment of CMOS image sensors are simulated, and the prepared P-type silicon samples are bombarded. A high-frequency C-U testing device is employed to test the high-frequency C-U curves of the samples before and after bombardment. Based on the test data and analysis using the Shockley-Read-Hall(SRH) theory and minority carrier transport equations, it is concluded that the internal positive charge deposition in the passivation layer and the increase in defect state density at the silicon interface caused by electron bombardment are the intrinsic reasons for the reduction in the electron multiplication factor of this type of image sensor.