An air gun with a quick-releasing device has a body, a head tube attached to the body and a detachably functional tip, wherein the quick-releasing device is attached to a distal end of the head tube to secure the detachably functional tip. The quick-releasing device has a slit defined in the distal end of the head tube to communicate with an inner space of the head tube, a locking sheet movably received inside the slit to partially enter the inner space of the head tube, and a locking lever pivotally mounted on the head tube to drive the locking sheet to move in a vertical direction. Thereby, the locking sheet is allowed to leave the inner space of the head tube when a user presses the locking lever to enable easy release of the attached functional tip and then to exchange the original tip for another functional tip.