The design flood in a mountainous small catchment was calculated by using distributed model, regional instantaneous unit hydrograph, and deducted formula respectively based on the ratio of peak discharge to catchment area, and the calculated results were further compared with the measured peak discharge of 100-year flood to determine the applicability of the above-mentioned three methods. Puhe river system in Qijiang, Chongqing was selected as a case study. Conclusions are drawn as follows 1) deducted formula is more accurate and reliable for catchment with an area smaller than 10 km² as steep river slope makes the runoff yield and concentration condition consistent with the assumption of the deducted formula; 2) distributed model and regional instantaneous unit hydrograph are both applicable to catchment with an area between 10-100 km², with the result of the former closer to measured result; in such catchment of long channel and gentle slope, the hydrodynamic dispersion can be regarded as uniform diffusion in space, and the computation grid units are strongly continuous, well reflecting the real design flood process; 3) distributed model is recommended as the preferred method when catchment area is larger than 100 km², as it deviates to the smallest extent to the measured value among the three methods. The conclusions will be helpful to the selection of a reasonable calculation method.