As an important performance index of the low-light-level image intensifiers, the signal-to-noise ratio can determine the detection limit under the low-light conditions. Therefore, an accurate evaluation of the signal-to-noise ratio is helpful to grasp actual working status of the low-light-level image intensifiers. In the photoelectric image transmission process, the quantum noise of the photocathode, the particle noise of the microchannel plate and the phosphor screen will add noise to the transmission chain of the image, thereby reducing the signal-to-noise ratio of the image. Laboratory testing methods for the signal-to-noise ratio of the low-light-level image intensifiers are relatively mature, and the photomultiplier tube is usually used as the core detection device to detect the signal and noise of the output light spot image. The test light source specified in the laboratory test of the signal-to-noise ratio is 2 856 K light source A. Due to the difference of spectral distribution between the night sky light and the light source A, the laboratory measurement results of the signal-to-noise ratio cannot accurately describe the signal-to-noise ratio of the low-light-level image intensifier under actual night sky light conditions, and the actual night sky light spectrum distribution is relatively complicated, which is difficult to be simulated by the laboratory test light source. In response to this problem, the theoretical calculation model of the output signal-to-noise ratio of the low-light-level image intensifier is deduced based on the photocathode and night light spectrum matching relationship and the theory of light quantum noise fluctuation. After verifying the model based on the parameters of the typical low-light image intensifier, the output signal-to-noise of the two under the radiation of three kinds of night sky light and test light source with the same illuminance is calculated based on the model. The comparative analysis results show that:1) Under the radiation of three kinds of night sky light, the actual signal-to-noise ratio of the super second and third generation image intensifiers is far greater than the test results under the radiation of test light source with the same illuminance in the laboratory. The reason is that compared with the light source A, the low-light image intensifier has a larger sensitivity conversion coefficient for the reflection spectrum of the actual grass scene. It further proves that the laboratory test results of the signal-to-noise ratio of the image intensifier cannot describe its actual working status under these three kinds of night sky light conditions; 2) Under the radiation of three kinds of night sky light, The difference in signal-to-noise ratio between the third generation and the super second generation is greater than the laboratory test difference, and this difference increases with the increase of illuminance. Therefore, the third-generation low-light image intensifier has a greater signal-to-noise ratio advantage in the actual working environment of the night sky light radiation. In addition, the theoretical calculation of the signal-to-noise ratio under extremely low illuminance levels is helpful to broaden the illuminance range for the evaluation of the signal-to-noise ratio. Under the condition of clear starlight, the illuminance of the reflected light from the actual scene reaching the cathode surface of the low-light night vision system is 10^-5 lx. Under the condition of cloudless clouds, the actual illuminance of the photocathode surface even reaches the level of 10^-6 lx. The current laboratory test conditions are difficult to describe the detection limit performance under these two conditions. Therefore, the theoretical calculation under the extremely low illuminance level of 10^-6 lx helps to broaden the illuminance for the signal-to-noise ratio evaluation. The research in this thesis lays a theoretical foundation for the evaluation of the signal-to-noise ratio of the low-light-level image intensifiers under the night-sky radiation conditions where the spectral distribution is more complex. At the same time, it also theoretically discussed the signal-to-noise ratio under the extremely low illumination levels.