An actuating mechanism used in an air gun is disclosed to include a hammer, a hook member hooked on the hammer for moving the hammer to the striking position, and a movable block supported on said spring member below a slide of the air gun. The movable block is turned to compress the spring member and to force a first stop edge thereof into engagement with a pivot in the air gun, and the hook member is forced to turn the hammer backwardly downwards to a hammering position when the user moved the slide to the rear side of the air gun. The hammer is disengaged from the hook member to strike the movable block, and the spring member returns the movable block to force a second stop edge of the movable into engagement with the pivot when the user triggered the air gun.