The invention provides a flood forecasting method and device. The method comprises the steps that multiple values of preset initial optimization parameters, multiple groups of empirical values of preset relevant parameters corresponding to each value and multiple groups of empirical values of corresponding preset forecast parameters are acquired, and a relation curve of the preset forecast parameters relative to the relevant parameters corresponding to each value of the preset initial optimization parameters is obtained; multiple values of preset optimization parameters and multiple groups of empirical values of a first kind of corresponding preset forecast parameters whose influences are not considered and a second kind of corresponding preset forecast parameters whose influences are considered are acquired, and a relation curve of the second kind of preset forecast parameters relative to the first kind of preset forecast parameters corresponding to each value of the preset optimization parameters is obtained; according to a preset station and a preset forecast duration, the preset relevant parameter values, the preset initial optimization parameter values and the preset optimization parameter values are acquired; according to the relation curves, a final preset forecast parameter value is obtained. According to the method, multiple optimization parameters are considered, and the obtained forecast result is accurate.