Abstract: The problem of comprehensive forecast of flood-waterlogging year of the Qiantang River Basin is discussed. It has been known that flood-waterlogging year results from comprehensive action of various physical factors. Because these factors have the laws of trend, period and arbitrary change, there are trend, period and arbitrary change components in flood-waterlogging series, hence comprehensively reflecting the process of long period change of flood-waterlogging series Y(t)by means of trend item T(t), period item P(t)and arbitrary item A(t)has been put forward: Y(t)=T(t)+ P(t)+ A(t)(t=1, 2,…). The result indicates that it is feasible that the process of long period change of flood-waterlogging year of the Qiantang River Basin is researched by means of comprehensive method, and the effect of comprehensive forecast model put forward is comparatively good. Therefore comprehensive forecast by means of change process of flood-waterlogging series itself is a comparatively effective way, which is of actual significance for flood-waterlogging prevention and sustainable development in the basin.