Precipitation washout is a common and effective measure to restore the storage capacity of reservoir. In order to study erosion effects under the condition of different boundary conditions, control water level and flood duration in later sediment retaining period of Xiaolangdi Reservoir flood duration, four groups of precipitation washout tests were conducted. The paper analyzes reservoir erosion effects of the different group. In this paper, the result show that, with more discharge and sediment, lower water level, longer duration and higher deposition surface, the more scour depth and better effect will be obtained. Based of experiment data, the relationship between output sediment discharge rate and input discharge, sediment concentration and water gradient is obtained and verified by measured data of Sanmenxia Reservoir, and the relationship between cumulative erosion and input water and sediment amount and water level before dam are obtained. INTRODUCTION Research of operational mode of Xiaolangdi Reservoir for flood control and sedimentation reduction during later sediment retaining period refer "multi-year sediment regulation and man-made precipitation washout at right occasion". Man-made precipitation washout at right occasion refers to under the favorable reservoir boundary condition, when suffer a large flood process, decrease reservoir water level, scour reservoir sediment, recover storage capacity, prolong the reservoir sediment storage life. At present, the operation of Xiaolangdi reservoir has entered the late sediment retaining period, recovery capacity is an important content. This paper studies the influence of boundary conditions, water and sand conditions, control water level and other factors on the scouring effect, and puts forward the relationship between sediment transport rate and the influence factors.