A waterproof structure of a respiratory tube is provided. The waterproof structure comprises a hollow body and a lid. The hollow body has a first opening end and a second opening end opposing to the first opening end. The lid pivots onto the hollow body at the first opening end. Before the lid is immersed into liquid, the gravity of the lid can keep the lid at a certain position so that the respiratory tube is well-ventilated. After a part of the lid is immersed into the liquid, the buoyancy provided by the liquid forces the closing surface thereof to substantially seal the first opening end and prevent liquid from going into the tube.