In the design of high rockfill dams on the deep and thick gravel-sand foundation with earth core, double anti-seepage walls will be more frequently used. The conjoining area of the anti-seepage soil body and the concrete wall is the key part to prevent seepage failure, and the optimization of structural arrangement in this area is the emphasis in a design process. In this paper the process of design scheme of the conjoining area in the Pubugou Rolled Earth-rock Fill Dam is presented, and the research focused on the pore pressure and seepage gradient character of two optimum schemes, i.e. one is that the cutoff wall and the gallery are entirely packed by clay, and the other, those are partially packed on the top by clay. It is indicated that the two schemes are both suitable. Seepage gradient in the conjoined area is inhomogeneous around the concrete structure. Seepage gradient at the top of the structure is much bigger than that at the outlet of the earth core base. Using high plastic clay package only on the top of the concrete structure is favorable to the settlement of the earth core and the construction schedule, so it is recommended to be used in the design.