A diving snorkel-mounted water stopper includes a sleeve unit defining a plurality of circumferentially spaced receiving chambers and a plurality of separated air valve openings corresponding to the tops of the receiving chambers; a ring-shaped housing externally located around the sleeve unit and having a plurality of air intakes formed at positions separately corresponding to the receiving chambers and the air valve openings; a plurality of float units separately disposed in the receiving chambers to normally locate below the air valve openings, and a cap closed onto a top of the ring-shaped housing. The separated air valve openings are independently controllable to open or close by the float units arranged in one-to-one correspondence thereto. When one of the air valve openings is closed, other air valve openings are not affected and air from above water surface can still be supplied via the remaining open air valve openings to a diver.