A powder-actuated fastener-driving device includes a barrel mounted in a housing. The barrel has a barrel wall formed with a through hole communicating with a radial vent hole. The through hole and the vent hole constitute an exhaust passage. A driving piston is mounted movably within the barrel, and has a rear end portion cooperating with an inner annular surface of the barrel wall to define an annular gap therebetween. A power adjusting unit includes an adjusting member disposed movably in the housing, and operable to move a control end portion thereof within the through hole so as to control an effective open area of the exhaust passage such that combustion gases discharge from a firing chamber defined between the driving piston and a rear end portion of the barrel through the gap and the effective open area, thereby controlling venting of combustion gases from the firing chamber.