Abstract: In order to study the influence of filling method on the stability of cofferdam which has grain-containing clay on the inside and block stone on the outside based on the 1 st earth-rock cofferdam of Yongning Water Conservancy Project,this paper firstly proposes the filling method of"block stone dike first,grain-containing clay follow-up"and determines the formula for calculating the intrusion distance of grain-containing clay according to the geometric relationship between the slope of block stone and the grain-containing clay and the followup distance. Secondly,based on the fluid-solid coupling theory,it studies the cofferdam working state influenced by seepage field and stress field under different drainage velocity of the foundation pit and combined ABAQUS with finite element strength reduction method to calculate the cofferdam in which grain-containing clay has intruded to the bottom of the dike. The results show that with the increase in reduction coefficients,the plastic zone develops from the bottom of the outside and gradually forms a penetrating plastic zone,and the maximum deformation appears at the bottom of the outside. The stability of the cofferdam is less influenced by the follow-up distance of grain-containing clay when it is more than or equal to the critical value of 16.22 m.When the follow-up distance is less than 16.22 m,the amount of grain-containing clay intrusion into the bottom of the dike is too large,which leads to the decrease in the bearing capacity of the cofferdam,and the stability of the cofferdam is threatened. In order to ensure the safety and stability of the cofferdam,it is recommended to control the follow-up distance of the grain-containing clay to be around 17 m.