Technical Abstract: Grain protein concentration is an important quality index, and formation of grain protein largely depends on pre-anthesis nitrogen assimilation and post-anthesis nitrogen remobilization. In this study, we developed a simplified process model for simulating plant nitrogen uptake and remobilization and grain protein formation. Six field experiments involving different years, sites, varieties, nitrogen rates and irrigation regimes provided the data for model building, genotypic parameter estimation, and model validation. Using physiological development time (PDT) as the time scale of developmental progress and cultivar-specific grain protein concentration as a genotypic parameter, the dynamic relationships of plant nitrogen accumulation and translocation with environmental and genetic factors were quantified and incorporated into the model. Pre-anthesis nitrogen uptake rate changed with PDT in a negative exponential relationship, and post-anthesis nitrogen uptake rate changed with LAI in an exponential equation. Post-anthesis nitrogen translocation rate depended on the plant nitrogen concentration and dry weight at anthesis and plant nitrogen concentration at maturity. Nitrogen for protein synthesis in the grain was derived from two sources: the nitrogen pre-stored in vegetative components before anthesis and then remobilized after anthesis, and the nitrogen uptake after anthesis. Model validation of final grain protein concentrations at maturity with the observed data exhibited reliable performance for different cultivars, ecological sites, nitrogen rates and irrigation regimes. These results suggest that integrating the grain protein formation model with other rice growth models will aid in predicting grain protein concentration and grain protein yield of rice under various environments and genotypes. Future studies are necessary for testing and improving the performance of the model for grain protein formation under diverse conditions, while research is continuing on digitalizing the whole rice production system.