The simulation of cottonseed (Gossypium hirsutum L.) growth is still an area of great uncertainty, especially in the process of cottonseed quality formation. A simple process-based model was developed to predict cotton bollmaturation period and simulate cottonseed biomass accumulation, protein, and oil content. The cotton boll maturation period module took solar radiation and N nutrition factors into account in addition to temperature and variety maturity profile. Based on the hypothesis that the accumulation of biomass, oil, and protein are mainly sink-determined, the model was developed by considering parameters of cultivar characteristics, weather (temperature and solar radiation), and crop management variables (precisely N supply). The subtending leaf N concentration of cotton boll was simulated by a new semi-empiricalmodel, and worked as the direct indicator of the N nutrition effect on cottonseed growth and development. The model was calibrated using data obtained in experiment conducted in Nanjing (the lower reaches of Yangtze River Valley) in 2005 and 2006. The model was then tested using two field experimental data sets. One was obtained in Nanjing, China in 2007, and the other in the Yellow River Valley (Xuzhou and Anyang) and the lower reaches of Yangtze River Valley (Huaian), China in 2005. The simulated values of boll maturation period by the model were very consistent with the observed values, with root mean square error (RMSE) lower than 3 days. The RMSE of cottonseed dry weight, protein content, and oil content predictions were 8.9 mg seed_1, 2.19%, and 2.71%, respectively. The result showed that the model is sufficiently robust to predict the cotton boll maturation period, cottonseed dry weight, and quality in wide range of conditions. It is not only a necessary component of cotton growth model, but also provides a good platform for further study in modeling cottonseed protein and oil yield.