The approximate inflow flood and interval flood are obtained by linear interpolation，then the derivative of flood regulation equation is introduced to construct Hermite polynomial to obtain the outflow flood process，and the corresponding flood of the downstream section of the reservoir can be obtained by regional composition.Then，according to the maximum safe flood passing capacity of the section，the weight parameters of linear interpolation can be solved，which can quickly determine the inflow and interval flood peak under the condition of satisfying the downstream safety flood discharge conditions，thus estimating the flood control protection capacity of the free overflow reservoir to the downstream.Hermite interpolation method can reach cubic algebraic precision，and the flood process adopts linear combination of two typical processes，so that the estimation method has higher precision and larger applicable flood variation range，which can guide subsequent relevant calculations in engineering design，and can also be used as a reference for flood control decision-making of free overflow small reservoirs．