The Miaoyuanxi River, located in Quzhou City of Zhejiang province, is a typical mountain stream. The river has characteristics ofplentiful rainfall, steep gradient, short source and swift flow, sudden flood rising and sharp dropping, quick travel speed, and great peak discharge. The triangulated irregular network and spatial analysis function of geographic information system(GIS) were analyzed to realize interpo -lation of the isoline of hydrological statistical parameters. According to the hydrological characteristic parameter derived from GIS, the averagepoint rainfall at different duration was calculated. In view of conversion coefficient of point rainfall into area rainfall, the average area rainfallwas obtained. By frequency analysis, design rainfall at different frequency was calculated. The instantaneous unit hydrograph was applied tocarry out convergence computation and infer design flood hydrograph and peak flow, which provide basis for drawing mountain flood risk mapand establishing security system for small watershed flood control and flood evasion.