Abstract: The method of anomaly correlation coefficient was used to analyze the relation between monthly runoff and prophase meteorological factors, and the meteorological factors set of monthly runoff prophase was established. The BP network based on the meteorological factors was used to set up the model of monthly runoff on Ertan hydropower station. The monthly runoff forecast in low water season and flood season indicated that the model had high forecasting precision, and the results showed that the forecast was in good agreement with measured value. The forecast model of monthly runoff based on physical meteorological factors was feasible.