Effective rainfall accurate determination can promote the technology development of planning crop irrigation, irrigation water management, irrigation and drainage design and highly efficient use of water resources. However, the effective rainfall will be affected by many factors, such as the characteristics of rainfall, characteristic of the crop’s type and crop water consumption, soil characteristics and the way farming farmland management. In addition, geographical location, climatic conditions and other factors will also affect the calculation of effective rainfall. Thus, a reasonable calculation method of effective rainfall should be determined based on the specific conditions of the region. In this paper, based on the meteorological data of Changzhi in Shanxi, the effective rainfall in the spring maize growing period was calculated by the method of water balance, under different hydrological conditions and different irrigation. And then this paper applied this method to analyze the effective rainfall of spring maize and simulate daily soil water content. The results show that: with the increase of rainfall, effective rainfall decreases. With the increase of irrigation amount, effective rainfall gradually reduced. With the increase of crop water requirements, effective rainfall gradually increased. Therefore, this paper analyzes the relationship between effective rainfall and rainfall, irrigation and crop water requirement. The correlation coefficient is 0.802. It is closely related. The simulated and measured values of the growing period of spring maize daily soil moisture better fit, weather in the irrigation or non-irrigation conditions. This verified the calculation method of the effective rainfall.