The purpose of this study is to examine a method for predicting plant photosynthesis and photomorphogenesis using spectral irradiance simulation. We compared the measured spectral irradiance in a greenhouse with the simulated values in a 3D greenhouse model to validate calculation accuracy. Using the measured and calculated spectral irradiance, we calculated the photosynthetic effective photon flux density and red: far-red ratio, which have correlations with photosynthesis and photomorphogenesis. Additionally, we studied a method of appropriately abstracting and modeling plant canopy to reproduce the light environment in the shade of the plants. The prediction accuracy of the PPFD outside and inside the greenhouse was high; further, we demonstrated that the angle and area of the individual leaves affected the calculation of spectral irradiance in the shade of the plants. The prediction method for PPFD and R/FR shown in this study will be applicable for indoor greenery planning.