The invention discloses an injection molded bottle cap, which is fine in sealing effect and comprises a cap top and a peripheral wall integrated with the cap top. A downward extending annular inner plug is arranged on the inner surface of the cap top, the inner surface of the bottle cap is provided with an annular projection which is positioned at the intersection of the cap top and the peripheral wall, and the projection and the inner plug are in interference fit with the outer wall and the inner wall of a bottle mouth respectively. The outer wall of the bottle mouth can be further sealed by the annular projection, so that a two-layer leakage-proof system is formed, and leakproofness of the bottle cap is improved.