Observations indicate that global radiation reaching the Earth’s surface has gradually decreased over the past several decades (i.e., the “global dimming” phenomenon), while the fraction of diffuse radiation in global radiation has increased. It has been found that the increase in diffuse fraction significantly increases photosynthetic production in the canopy, but the mechanism has not been clarified. A three-dimensional light distribution model considering the geometry of incident radiation was employed in this study. This model was used to simulate the light distribution and potential photosynthetic production of rice canopy. The results indicate that the potential photosynthetic production of rice significantly increases in response to an increase in the diffuse fraction as long as global radiation does not decline greatly. The “fertilization effect” of diffuse radiation results from the reduction of leaf area with photosynthetic photon flux density (PPFD) below the light compensation point in response to an increasing diffuse fraction, and an increase in the leaf area with PPFD on the linear part of the light response curve. The increase in the diffuse fraction results in a significant increase in the photosynthetic rate of the upper canopy. High-performance computation is an easy-to-use and economic approach to overcome the computational constraints of the model.