A charging system, a charging method, and a power adapter are provided. The power adapter includes a first rectification unit, a switch unit, a transformer, a second rectification unit, a first charging interface, a sampling unit, and a control unit. The control unit is configured to output a control signal to the switch unit, and adjust a duty ratio of the control signal based on a voltage sampling value and/or current sampling value sampled by the sampling unit, in which a voltage of a third pulsating waveform output from the second rectification unit meets the charging requirements.