A charging system, a charge method, and a power adapter are provided. The charging system includes a battery, a first rectifying circuit, a switch circuit, a transformer, a second rectifying circuit, a sampling circuit, and a control circuit. The control circuit outputs a control signal to the switch circuit, and regulates a duty cycle of the control signal according to the current sampling value so that the voltage with the third pulsating waveform satisfies a requirement of charging the battery.