The present disclosure discloses a charging system, a charging 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 second rectifier, a sampling unit, a control unit and a first isolation 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 value and/or voltage 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. The terminal includes a battery. When the terminal is coupled to the power adapter, the third voltage is applied to the battery.