The commensurability indicates a certain order existing in natural phenomena. The commensurability (approximate periodicity) is the expansion of periodicity. A method of the arithmetic information forecasting, in terms of the concept of the commensurability of hydrologic disasters and their grey period extension, is proposed as a possible approach of longterm hydrologic forecasting. The floods and droughts over the middle and lower Yangtze River can be forecasted by means of the ternary commensurable formula in direct extrapolation and its physical mechanism can be analyzed on the basis of the network structure of commensurability information system. Predictions show that big deluges may occur over the middle and lower Yangtze River around 2008, 2013，and an extraordinary flood may occur over the upper Yangtze River around 2009 and the severe drought appeared in 2007. The method has a strong physical basis and a high mimic accuracy, but the extensional forecast accuracy will wait to be examined by facts.