The study of the relationship between climatic productivity and maize yield, and the relationship between soil water content and precipitation, and the relationship between soil water content and the sum of precipitation and initial soil water content in the Northwest China is showed that the correlation between climatic productivity of precipitation and maize yield, and the correlation between soil water content and the sum of precipitation and initial soil water content in local area are obvious. Based on them, Miami model of precipitation was improved by soil water content of sowing time and precipitation during maize growing period. The improved Miami model of precipitation is better in operational dynamic forecasting of maize yield trend every month. And also, it is practicable in assessment of precipitation influence on growth and development and dry matter accumulation of maize using this method in the Northwest China. But it is not suitable for the irrigating areas, because temperature and solar hour etc. Were not considered in this method.