The present disclosure discloses a charging system and a charging method and a power adapter. The system includes a battery, a first rectifier, a switch unit, a transformer, a second rectifier, a first charging interface, 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 sampled by the sampling unit, such that a third voltage with a third ripple waveform outputted by the second rectifier meets a charging requirement of the battery.