A piston assembly of an air pump is provided herein. The piston of the piston assembly has a through exhaust channel along the direction of the piston's back-and-forth movement. The exhaust channel is made up of two aligned exhaust holes, and a space is preserved at where the two exhaust holes interface. A flexible valve piece is provided inside the space, which will flip upward and downward to block the exhaust holes when the piston moves downward and upward, respectively. When the air pump is turned off and the piston stops moving, the exhaust valve piece would be automatically restored to its un-bended state by its own flexibility, allowing the compressed air inside the cylinder to escape through the exhaust holes and around the exhaust valve piece inside the space.