The transformation of dam-site flood series to corresponding reservoir inflow flood series by correlation analysis is needed when frequency analysis method is used to estimate reservoir inflow design floods. The commonly used linear regression method assumes that these two variables have linear relationship and reservoir inflow flood series follow normal distribution, which may not be conformed to the actual situation. This paper proposed a new interpolation method for reservoir inflow flood based on Copula function, which was adopted to construct the joint probability distribution and conditional probability distribution of dam-site flood and reservoir inflow flood. The conditional most likely values and confidence intervals of reservoir inflow flood are obtained given the value of dam-site flood. The application in the Three Gorges Reservoir indicates that the reservoir inflow flood values estimated by linear regression method are obviously smaller than those by the Copula method when the flood magnitude is large, even are not within the 90% confidence interval when dam-site flood with rare frequency occurs. The proposed method can reflect the inherent relationship between dam-site flood and reservoir inflow flood and give both point and interval estimates of reservoir inflow flood and evaluate the uncertainty of estimation quantitatively.