There are only a few results from application of Tank model real time flood forecasting. The purpose of this research is to apply tank model in real time flood forecasting for large scale basins. This paper introduced the method of developing flood forecasting model for river system with tank model. The Main stream and tributaries above Cuntan Station were separated into many reaches according to the spread time of flow, and every reach is separated into many unit reaches. Runoff of every unit reach was computed with tank model, and flow of stream was computed with continuous Muskingum. The precision of the river system flood forecasting model is above 85%, and the result satisfies real time flood forecasting.