Based on photon-transport theory and the theory of electron-semiconductor interactions, a model for the formation mechanism of gain noise caused by secondary electron fluctuations in the electron-multiplying layer of electron-bombarded complementary metal-oxide-semiconductor (EBCMOS) devices is established. By performing a Monte Carlo simulation, the signal-to-noise ratio (SNR) of secondary electrons is calculated to analyze the effect of system parameters, including incident electron energy, passivation-layer type and thickness, electron-multiplying layer thickness, and doping concentration, on gain noise. The results indicate that by using Al₂O₃ as the passivation-layer material and implementing a series of measures such as reducing the passivation-layer thickness, doping concentration, electron-multiplying layer thickness, and operating temperature, the SNR of the electron-multiplying layer can be effectively improved to 75.35 dB. The findings of this study provide theoretical guidance for preparing the electron-multiplying layer in EBCMOS devices.