Flood disaster brings serious pecuniary loss to people. Small basin floods usually cause instant devastating disaster to downstream because of fast concentration. Therefore, it is necessary to take effective measures to control floods in small watershed. This paper based on the characteristics of the small watershed like mountain -plain -polder mixing area to analyze and compare methods of calculating peak flow, and recommend using instantaneous unit hydrograph method and carrying on some research on a combination of design flood in small watersheds. Then, the upper Tuanjiehe River Basin was taken as an example to calculate its design flow of 20-year flood control (472.60 m3 /s).