A system and a method for mixing cabin and outside air of a vehicle air-conditioner may include a housing having cabin and outside air inlets for ventilating the cabin and outside air, a first door part to selectively cover at least a portion of the cabin air inlet and/or a portion of the outside air inlet to adjust an open value of each of the inlets, a second door part to selectively cover the remaining portion of the outside air inlet to adjust the open value of the outside air inlet, a cam part rotatable in the housing and formed with first and second slots to which the first and second door parts are slidably coupled such that rotation angle of the first or the second door part is varied according to rotation of the cam part, and a controlling unit controlling the rotation of the cam part.