A combined model was established for simulating canopy photosynthesis based on the data observed with portable Licor 6200 photosyntometer. The architecture of crop canopy was fully considered in the model,and the model can simulate instantaneous photosynthesis.Results of the validation of the model by the observation data measured with GHX 305 CO2 analyzer showed that the model can simulate photosynthesis rate of crops in field fairly well.Results of the simulation of the totals of daily CO2 assimilation with the model showed that greater deviations existed in the basic parameters of FAO productivity model.The basic parameters in FAO productivity model were reconstructed in this paper,which will provide theoretical basis for estimating crop productivity more accurately.