The invention relates to a security system with power saving feature and device comprises an electric magnet and a corresponding iron plate. The electric magnet is connected to a G-sensor module and the iron plate has a pressing face to contact with the G-sensor module. When the door is opened, the electric magnet does not supply power; when the door is closed, the electric magnet with electromagnetic attraction attracts the iron plate and the pressing face presses the G-sensor module. That is, the electromagnetic lock usually stays in a low-energy attraction state; however, when the G-sensor module is triggered, the electromagnetic lock returns to a normal lock state for achieving power saving effect and control of the external force detection improvement.