A method for quickly replacing an air outlet pipe of an air blow gun includes providing a gun body, a C-shaped clamping ring, a rotation knob and a nozzle head, and rotating the rotation knob to rotate the clamping ring, moving the oblique guide faces of each of the clamping jaws of the clamping ring to touch the resting portions of each of the arcuate slots of the mounting head of the gun body, pressing outward each of the clamping jaws of the clamping ring, releasing the clamping jaws of the clamping ring from the locking groove of the nozzle head, detaching the nozzle head from the clamping ring, and detaching the nozzle head from the mounting head of the gun body. Thus, the nozzle head is detached from the gun body quickly.