The invention discloses a super-long-term drainage basin/regional flood forecasting method. The method comprises the steps of (1) building a drainage basin/regional forecasting model; (2) building a specific place forecasting model; and (3) performing comprehensive identification on the forecast. In the step of building a drainage basin/regional forecasting model, series samples of extremely serious floods in a drainage basin are applied, calculation is performed by applying ternary and quinary commensurability forecasting formulas, and forecasting achievements are acquired; sample sequences emerged in a forecasting achievement commensurability formula are counted, a sample with the highest occurrence frequency is selected, spatial features and magnitude features of the sample act as forecasting achievement features, an idea of information forecasting is reflected, and non-accidental information is extracted from the forecasting formula.