The invention discloses an energy-saving alternating-current demagnetization device and method used for ferromagnetic pipe electromagnetic testing. The device comprises a demagnetizer, a current controller, a photoelectric sensor and a magnetic measurement sensor. A negative feeding method is adopted, the intensity of the surface magnetic field of a workpiece is monitored by using the magnetic measurement sensor, the intensity is fed back to the current controller in time when the intensity of the magnetic field reaches a demagnetization requirement, the increase of demagnetization current is stopped, and if load (namely no workpiece) exists, current is stopped conveying to the demagnetizer. Therefore, the purpose of automatically controlling the demagnetization current is realized, and the device has the beneficial effects of guaranteeing the demagnetization effect, reducing the energy consumption and protecting the equipment reliability.