The invention discloses a flood forecasting method based on synthetic flow and shape fitting and relates to the technical field of hydrological simulation and forecasting. The method comprises the steps that a correlation between target cross section flow and upstream cross section flow is inquired and forecasted; distributed hydrological model modeling is conducted on forecasting drainage basin,and obtained runoff generation and confluence parameters of each calculation unit in a distributed hydrological model are calibrated; according to flood peak data of each time and flood volume data ofeach time of a forecasting target cross section, the corresponding relation of the flood peak and the flood volume is fit; corresponding exponential-type water subsiding curve parameters in the actual water subsiding process are inquired; a flow forecasting result of the forecasting target cross section in the confluence forecast period is obtained through flow synthesis; the forecasting flood volume and the forecasting flood peak of the forecasting target cross section in the flood on the spot are acquired; the corresponding flow forecasting result which meets a preset requirement is used asa final result of flood forecasting of each time. According to the flood forecasting method, the forecast period of flood forecasting is prolonged, so that the forecasting result is more reasonable and precise.