The invention discloses a river channel flood discharge early warning method based on hydraulic simulation, and relates to the field of river channel flood routing and early warning. As reservoir flood discharge and river channel natural flow processes have obvious difference, on the basis of constructing a one-dimensional hydraulic model for a river channel, each obtained fracture surface water level sequence is simulated according to the model, and the flood rising time, flood peak reaching time and corresponding rising water level and peak value water level due to reservoir flood dischargeare automatically identified and extracted. The method is suitable for hydraulic simulation of the river channel, flood propagation time and magnitude prediction, has universality, is simple and easyto operate, can calculate flood propagation process according to a drainage plan of a hydraulic building or hydrological prediction of a natural river channel, and provides early warning information for disaster prevention and reduction of a downstream area.