In general,stage-discharge relationship influenced by fluctuation rate of flood is time sequence loop curve in natural river,but the correction factor method exists some difficulty in data processing by computer.On the base of mathematical analysis of the formula of diffusive wave,this paper puts forward the direct method for fitting loop curve of stage-discharge relationship for using computer.The paper introduces the principle of the method and gives out its calculation steps.The precision and effectiveness of this method are demonstratde by numerical examples.