The invention discloses a power supply cable prolonging device for a tunnel heading machine. The power supply cable prolonging device is characterized by comprising an A-segment cable circuit, a B-segment cable circuit and at least one C-segment cable circuit which are electrically connected in sequence, wherein the A-segment cable circuit consists of a transformer, a switch and a first three-way high-voltage cable joint which are electrically connected in sequence; the B-segment cable circuit consists of a second three-way high-pressure cable joint, a first high-voltage cable and a heading machine which are electrically connected in sequence; the C-segment cable circuit consists of a third three-way high-voltage cable joint, a second high-voltage cable and a normal high-voltage cable joint which are electrically connected in sequence. The invention further provides an operation method of the power supply cable prolonging device. The device has a reasonable structure, can be used for prolonging a high-voltage cable in a power-on state, is low in cost, and is convenient to use. The operation method is scientific and reasonable, and has high adaptability and a good effect.