An air pump includes a mouthpiece, an adjustment mechanism, and an air cylinder mechanism that includes a small-diameter cylinder permitted to move relative to the adjustment mechanism. When the small-diameter cylinder is moved away from the adjustment mechanism, air from external environment is sucked into a first space of the air cylinder mechanism. The adjustment mechanism is operable to switch between a singular pumping state, where air in the first space is released to the external environment when the small-diameter cylinder is moved toward the adjustment mechanism, and a dual pumping state, where the air in the first space is injected into the mouthpiece when the small-diameter cylinder is moved toward the adjustment mechanism.