Abstract: In engineering planning and design, the P-Ⅲ curve is usually used to calculate the design flood. However, in recent years, global climate change has led to frequent record breaking of flood peak flows, so from the perspective of engineering safety, it is necessary to adjust the frequency curve so that it is more inclined to the high-value region. In the usual method, this adjustment is realized by changing the ratio of C_v /C_s . However, this method leads to distortion and deformation of the curve, thus deviating from the ideal shape. Based on this, a new method is proposed: based on the gamma function, the high-valued region is appropriately encrypted and the inverse function of the gamma function is utilized for automatic curve fitting. This method not only maintains the reasonable shape of the P-Ⅲ curve and ensures a better fit of the curve to the frequency points, but also realizes the offset of the P-Ⅲ curve to the historical high-value area, thus reflecting the flood risk more accurately. In addition, this method significantly improves the working efficiency and calculation accuracy, providing a more scientific and accurate basis for flood calculation for engineering design.