The main purpose of this study is to indicate the factors affecting hydrologic budget of rainfall, evaporation & discharge of Shihmen reservoir watershed. Four phenomena have been developed: 1. drought period (Nov. ~ Apr.) & flood period (May ~ Oct.) analyses, and 2. applying the hydrologic statistics methods, should involve: (1) moving mean method, (2) least square method, and (3) Fourier series method. The results can accurately describe the hydrologic characteristics of a watershed in details, flood hazards prediction on the different return periods also present.