An air pump comprises an outer tube, a distribution guide tube, and a spring. The outer tube is provided in the interior with a large cylinder and a small cylinder. The distribution guide tube is disposed at the lower end of the outer tube and is provided with two air chambers, one of which is in communication with the large cylinder and is provided with a pressure leaking slide block, two leakproof rings, and an annular groove located between the two leakproof rings for forming a duct. The spring is disposed in the air chamber for providing the pressure leaking slide block with a recovery spring force. Another air chamber of the distribution guide tube is in communication with the large cylinder and the small cylinder for discharging air to inflate an inflatable object.