A waterproof cover for an audio system includes a frame and a shield pivotally connected to the frame. The flame has two elongated recesses respectively defined in two opposed sides defining the through hole and an inclined trough sandwiched by the two elongated recesses and defined in a bottom side defining the through hole. The shield has two rails formed on two opposed sides thereof to correspond to and be movably received in the two elongated recesses, a water blocking flange sandwiched by the two rails and formed on a bottom side of the shield to correspond to and be received in the inclined trough and a flange formed on a top side of the shield to abut a top side defining the through hole such that a full coverage of the through hole by the shield is completed and water seeping from engagement between the shield and the frame is reduced.