In order to improve the utilization rate of rainfall in Nanjing City, rainfall data forecasted by the National Meteorological Center over a forthcoming four-day period and observed rainfall data from the Nanjing Meteorological Station for the period from June 26 to October 28, 2011 were collected. The accuracy of rainfall forecasts and their applications to rice irrigation decision-making were analyzed. The results show that the accuracy was high in forecasting light rainfall and no rainfall, and the calculated irrigation water could be applied with a low risk of wasting irrigation water. When moderate rainfall and heavy rainfall were forecasted, there was a high probability of the occurrence of rainfall, and two-day delayed irrigation could be applied. Supplementary irrigation could be applied if there was no rainfall or if the rainfall could not meet the requirements of crop growth.