The invention discloses a method for changing the size of a cavity to discharge tar in one way. The method comprises the steps that when a suction mouth and a cigarette inserting pipe relatively move, the size of the tar collecting cavity between the suction mouth and the cigarette inserting pipe is reduced; along with relative movement of the suction mouth and the cigarette inserting pipe, a smoke channel communicated with the tar collecting cavity is completely or partially cut off, the pressure of the tar collecting cavity is gradually increased, and a one-way tar discharge sealing piece disconnecting the tar collecting cavity from the outside is opened under the action of the pressure of the tar collecting cavity, so that the tar collecting cavity and the outside are directly or indirectly communicated to form a tar outlet channel. Under the action of the pressure, tar in the tar collecting cavity is discharged along the tar outlet channel, the tar is attached to a cigarette butt in a cigarette inserting opening, and the tar is taken away every time the cigarette butt is thrown away. The suction mouth and the cigarette pipe are reset, the size of the tar collecting cavity is gradually increased, communication between the tar collecting cavity and the outside through the one-way tar discharge sealing piece is cut off through the one-way tar discharge sealing piece again, and the smoke channel and the tar collecting cavity are communicated again.