A device for opening and closing a damper using a knob of an air vent in a vehicle includes: a wing knob coupled to a horizontal wing mounted in an outlet of an air duct such that the wing knob is slidably movable on the horizontal wing; a first rack gear formed on a rear surface of the wing knob; a transfer gear body meshing with the first rack gear and rotatably mounted on upper and lower vertical wing support frames; a cam gear coaxially and rotatably connected to a lower portion of the transfer gear body; a slider mounted to a bottom surface of the air duct such that the slider is movable in a forward or backward direction on the bottom surface of the air duct, wherein the slider moves forward or backward while contacting the cam gear at a time of rotation of the cam gear; second rack gears formed at both side surfaces of the slider; and damper gears meshing with the second rack gears and rotatably mounted on the bottom surface of the air duct. The damper is coaxially connected to the damper gears and disposed in the air duct.