In order to analyze the roughness coefficient of the sea buckthorn plant flexible dam,a field water flow test on the sea buckthorn plant flexible dam was conducted in the sea buckthorn plant flexible dam test base in Xiao-hua-shan Reservoir,H uaxian County,Shaanxi Province.The estimation formula of the sea buckthorn plant flexible dams roughness coefficient was derived by the river sediment dynamics theory,and the roughness coefficients of the sea buckthorn plant flexible dam were evaluated by the formula under non-submerged flow during some grow th stage.The results indicated that the sea buckthorn flexible dams roughness coefficient was approximately within the range of 0.04~0.09.The roughness coefficient of the sea buckthorn plant flexible dam was inversely deduced and calculated from the field flow test data by the river dynamics method,and it was found that the result was in accord with that calculated by the theory evaluation for mula.It was suggested that the theoretic estimation formula could be used to evaluate the roughness coefficient of the sea buckthorn plant flexible dam,and it was of great value to further study the effects and mechanism of the sea buckthorn plant flexible dam on sediment retention.