Two branch gullies of Chenjiagou,Wanzhou District in the Three Gorges Reservoir area were taken as the study object and the correlation of rainfall factor with runoff depth and peak flood amount was studied by grey relational analysis and correlative relationship.The data of over 10 typical rainfall events and the data of runoff in the small watershed firstly were processed by the line cutting method of base-flow separation.Results showed that by grey relation grade,the main influencing factors of runoff depth were found to be the rainfall amount(P),and compound factor(PI,the composite of rainfall amount and averaged rainfall intensity).In comparison of the grey relation grades for the two neighboring branch gullies,the grey relational grades of runoff depth responded to rainfall were different,but they had some common trends:the combined effect of peak rainfall intensity(I_60),and rainfall had a noticeable impact on runoff in the Three Gorges Reservoir area and the main influencing factor of peak flood amount was the compound factor(PI).