Abstract: FPAR (fraction of absorbed photosynthetically active radiation) is an important parameter for estimating plants net primary productivity of land ecosystem from remotely sensed data, and is also crucial for calculating canopy APAR (absorbed PAR) and estimating canopy Photosynthesis and the utility rate of luminous energy. Based on the theory of radiation transfer model, this paper modified the Simultaneous Heat and Water model to calculate FPAR vertical distribution in maize canopy and analyzed the relationships between FPAR and some parameters like maize canopy structure, solar zenith, soil reflectance, etc. The validation results using field measurements prove the model to be accurate.