The invention discloses a river flood forecasting method based on a discrete generalized Nash routing model. By introducing the detention storage curve, simplifying a generalized Nash routing model, and establishing a discrete generalized Nash routing model, a river flood forecasting method is provided based on this model. The method expresses the outflow process of a river lower section as a linear combination of the inflow at the beginning and the end of a period and the outflow of the current time and a plurality of previous time moments, and obtains a weight coefficient according to the detention storage curve. The river flood forecasting method based on a discrete generalized Nash routing model, because integrating some historical effective information, can improve forecasting accuracy, is simple and visual, and can be easily popularized and applied in engineering practice.