A power stapler in which the electromagnetic valve has a movable annular metal block which moves downwards to prevent the occurrence of a high striking noise when attracts the valve rod to strike the striking plate; the guard plate which stops staples in the magazine can be alternatively adjusted to the closed position to close the front end of the magazine, the open position to open the front end of the magazine for the loading of staples, or the mid-way position to partially close the front end of the magazine for permitting jammed staples to be removed from the magazine.