Partial duration flood (PDF) frequency analysis model was studied for expanding the flood information and enhancing the simulation accuracy of the flood. The basic theory of the PDF was introduced and the hypothesis was given that the annual occurrence number of the partial duration flood series is estimated by the Poisson distribution, the partial duration flood series are estimated by the generalized Pareto (GP) distribution, then the distribution of the annual maximum partial duration flood series and the calculation formula of the PDF return period could be obtained. A model goodness of fit test has been proposed for determining the threshold of the partial duration flood. The PDF model was applied in the flood peak frequency calculation for the Xiaojue Station in the Haihe River Basin. The results show that the goodness of fit test is an efficient and reliable method for determining threshold, and the annual occurrence number of the partial duration flood series follow the Poisson distribution. The design value of the flood peak based on the GP distribution is slightly larger than that on the P-Ⅲ distribution, which will be more secure when used in the design and the risk analysis of the hydraulic engineering.