Along with the development of remote sensing technique, performance demands of load become higher and higher. Especially the detection sensitivity of load is related to the ability of detecting target. How to meet and increase detection sensitivity of load is becoming a problem of developing high-performance remote sensing device. Based on the theories of photometry and radiometry, space target detection signal intensity is analyzed and calculated. According to classical detection theory, the model of detection sensitivity of space-based visible (SBV) sensor is derived based on the mathematical expression of target and noise. By substituting American SBV sensor parameters into the model, the error between calculation result (15.55 visual magnitude) and design specification (15.5 visual magnitude) of detection sensitivity is only 0.05 visual magnitude. This shows that the detection sensitivity model has considerable accuracy, and will provide a theoretical reference for space-based visible sensor research and design.