Abstract: In this article the potential productivity of climate is divided into three levels:solar radiation, radiation-temperature and radiation-temperature-water. Based on proportion of water supply to water requirement and by using RTW model the author has calculated the possible yield of maize in different seeding time. It is found that the calculated results fit the results of cultivated experiment successfully. It seems to support the idea that the theoretical seeding time in agronomy can be set up by theory of potential productivity of climate.