A power-off circuit includes a power unit (10), a power management unit (20), an integrated circuit chip (IC chip, 40), a switching apparatus (30), and a clamp circuit (70). The switching apparatus produces and transmits a power-off instruction to the power management unit. The power management unit produces and transmits a power-off notice to the IC chip in accordance with the power-off instruction. And the IC chip feeds back a power-off affirmation in accordance with the power-off notice to the clamp circuit. The clamp circuit receives the power-off affirmation and produces and transmits interrupted power-off signals to the power management unit, till the power management unit completely cuts off power supplied from the power unit to the IC chip.