[Objective] The effects of perennial grassland with different growing years and rainfall regimes on soil and water conservation were analyzed to provide a theoretical basis for the further development of biological engineering measures in hilly region of the Loess Plateau. [Methods] Data of total runoff sediment yield and 40 rainfall events collected from Suide Soil and Water Conservation Station from 1956 to 1960 were used to analyze the effects of perennial grassland on soil and water conservation in hilly region of the Loess Plateau. [Results] (1) The observed total runoff and erosion modulus of grasslands were smaller than those of farmlands. Runoffs of alfalfa and Melilotus decreased 20%~83% and 11%~88% respectively, as compared with the corresponding values of farmlands. Sediment reductions of grassland and farmlands were both more than 50%. (2) Based on K-Means clustering, 40 rainfall events were classified into three rainfall regimes according to the amount and intensity(30 min) of rainfall: regime Ⅰ had the characteristics of high rainfall, low intensity and occurred infrequently; regime Ⅱ had the features of medium rainfall, medium intensity and medium frequent occurrence; regime Ⅲ had the features of less rainfall, high intensity and high frequent occurrence. A large proportion of runoff and soil loss was caused by rainfall regime Ⅲ. Water reduction of grasslands was observed the best in regime Ⅱ of the three regimes. (3) Under regime Ⅲ, the benefit of water reduction increased with the increase of grass growth period, and soil reduction was small in earlier growth years and then increased afterwards. [Conclusion] Grasslands grew with the two grasses both had the benefits of water and soil reduction. The benefit were closely linked with rainfall regime. The benefit of water reduction and soil reduction were not always coincided.