In order to provide an effective tool for light distribution calculation within crop canopy on Windows platform, a light distribution calculation system derived from RGM model was developed using C++ language and OpenGL library. A maize canopy geometrical model was taking as the experimental example. Parameters were selected on the basis of feasible RGM model. The related methods were improved according to the characteristics of crop canopy. On the basis of 3-D maize canopy model, the light distribution situation of each organ facet could be calculated in real time by using parameters input via human-computer interaction. The system had user-friendly interface and was easy to operate. It only needed few input parameters and most parameters were meaningful in botany and agriculture.