The invention discloses a cutting portion of a tunneling machine. The cutting portion is characterized in that front-surface hobbing cutters are mounted on the front surface of a cutter head, a cutter arm support is mounted on the back surface of the cutter head, a first retractable cutter arm, a second retractable cutter arm, a third retractable cutter arm and a fourth retractable cutter arm are mounted on the cutter arm support, and chamfering hobbing cutters are mounted on the first retractable cutter arm, the second retractable cutter arm, the third retractable cutter arm and the fourth retractable cutter arm; the first retractable cutter arm and the second retractable cutter arm are respectively parallelly retractably arranged along the positive and negative directions of an X axis, the third retractable cutter arm and the fourth retractable cutter arm are respectively parallelly retractably arranged along the positive and negative directions of a Y axis, and the rotation axis of the cutter head is perpendicular to a plane formed by the X axis and the Y axis. The invention further discloses the tunneling machine and a cutting method. The cutting portion, the tunneling machine and the cutting method have the advantages that the diameter of the cutting portion of the tunnel machine can be varied, so that the diameters of the sections of tunnels can be varied, chamfering operation can be implemented, and the cutting portion and the tunneling machine are high in variable amplitude; the chamfering diameters are large, so that personnel can assuredly pass the tunnels, and maintenance and gear replacement can be implemented.