The objective of this study is to construct a suitable routing algorithm of HSPF (Hydrological Simulation Program Fortran) for the sloping areas in Ney-Shuang creek catchment, and to apply this algorithm in estimating the flood flow on the frequency rainfall base. The genetic algorithm is chosen to identify the appropriate parameters of HSPF model. The model routing is verified by comparing the evaluated results of 300 times Monte Carlo simulation with the verification events. By employing the proposed routing algorithm in frequent flood analysis, the confidence interval of flood discharge on the frequency base can be obtained. This result may have a significant influence in the management and the decision making of the sloping areas development.