Disclosed is an auxiliary mounting device of a pre-embedment grouting pipe in an impervious wall slot. An inner supporting frame is horizontally placed on supporting rods on an upper opening of a positioning frame, and in a positioning frame, a plurality of horizontal longitudinal connecting rods and a plurality of horizontal transverse connecting rods form a rectangular frame and then are perpendicularly welded to vertical rods to form a rectangular frame component; and the inner supporting frame is the rectangular frame component formed by perpendicularly welding a plurality of longitudinal reinforcing steel bars and a plurality of transverse reinforcing steel bars. The auxiliary mounting device of the pre-embedment grouting pipe in the impervious wall slot can effectively ensure precise positioning of the pre-embedment grouting pipe in the impervious wall slot, and prevent the phenomenon that the grouting pipe is impacted by side pressure in the concrete pouring process and mismatched, and the grouting and construction quality of a curtain is ensured.