Few studies have presented mathematical models and test methods for determining the radiation gain of AlGaN photocathode solar blind ultraviolet image intensifiers; thus, in this study, a mathematical model of radiation gain of a dual close-focused AlGaN photocathode ultraviolet image intensifier, with an effective diameter of .18 mm, is deduced. The mathematical model is based on the brightness gain of low-light-level image intensifiers; two values of incident irradiance are introduced to the photocathode and a visual function to the phosphor screen. The quantum efficiency of the photocathode, MCP(microchannel plate) current gain, and luminous efficiency of the phosphor screen are measured using standard laboratory equipment. The measured values of the parameters are substituted into the mathematical equation of the radiation gain, and the theoretical values of radiation gains of 10 samples are calculated. Additionally, a set of UV radiation gain test systems is modified; the systems are used to test the radiant gain of the aforementioned 10 samples and compare the mathematical theory and the actual measured values. The deviation between the two values is within 10%, and the validity of the mathematical model and test system are proven. The results of this study may serve as a reference for studies regarding high-radiation gain UV image intensifier technology.