The invention discloses a rainfall runoff forecasting system. An innovative rainfall runoff forecasting technical scheme is adopted, short, medium and long term rainfall runoff forecasting can be achieved, the real-time flood forecasting process can be displayed in forecasting, which is beneficial for accurately estimating the flood peak, the flood volume and the process of flood, the processes of discharge out of a reservoir and the water level are simulated in the forecasting process, short, medium and long term runoff forecasting scheduling including real-time flood forecasting of a user is achieved, and the problems that at present, the forecasting process of most runoff forecasting software is too short, and the medium and long term forecasting function is poor are solved. The system comprises a water regimen database, a user parameter configuration module, a rainfall runoff forecasting and calculating module, a forecasting result statistics module and a forecasting graph module. The system has the automatic forecast correcting function and is not provided with a gauging station and capable of achieving real-time rainfall flood forecasting as well, software and hardware cost of the system is greatly reduced, and the problem that most small and medium-sized reservoirs, hydropower stations and some large and medium-sized hydropower stations are short of medium and long term forecasting is solved.