An electric power-regulating system includes an electric-load end, a power-supply end, a power regulator, a DC-bus device, a flywheel energy-storage device, a switch device and a controller. The flywheel energy-storage device connects the DC-bus device. The switch device is connected between the flywheel energy-storage device and the DC-bus device. The switch device provides at least one current-flow direction between the DC-bus device and the flywheel energy-storage device. The controller controls the power regulator and the flywheel energy-storage device. When a voltage of the DC-bus device exceeds a operation range, the controller selectively limits the current-flow direction of the switch device. Further, when a voltage-bias direction of the switch device and the current-flow direction are the same, the controller allows a current of the DC-bus device to flow into or out of the flywheel energy-storage device in the current-flow direction. In addition, an electric power-regulating method is also provided.