Abstract: In response to the issue of increased luminance masking effect of nature light on fluorescence when outdoor natural light intensity increases, resulting in a decrease in the accuracy of oil leakage detection with fluorescence leakage detection, we elucidate that the luminance masking effect of natural light on fluorescence reduces the signal-to-noise ratio (SNR) of fluorescence image targets, and establish a fluorescence image target SNR model under natural light interference. The model takes into account the radiant emittance resulting from the interaction of natural light, fluorescence, and excitation light with the fluorescent liquid and solid surfaces in the imaging region, and the response characteristics of imaging sensors and the imaging rule of optical lenses. The effects of natural light intensity, surface reflectance and filter cutoff depth on the luminance masking effect are analyzed with this model. Experimental results show that an increase in natural light intensity, an increase in solid surface reflectivity and a decrease in filter cutoff depth will intensify the luminance masking effect. The image target SNR under different intensities of natural light shows that the difference between the model's estimated value and the measured value is less than 10%. This model can effectively evaluate the effect of natural luminance masking on the fluorescent image target SNR, and can be used for the quantitative evaluation of the application condition of natural light in fluorescence imaging systems.