The present disclosure discloses a charging system and method, and a power adapter. The system includes a power adapter and a terminal. The power adapter includes a first rectifier, a switch unit, a transformer, a synthesizing unit, a sampling unit, and a control unit. The control unit outputs a control signal to the switch unit, and adjusts a duty ratio of the control signal according to a current sampling value and/or a voltage sampling value, such that a second alternating current outputted by the synthesizing unit meets a charging requirement. The terminal includes a battery.