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, a control unit, a second charging interface, and a battery coupled to the second charging interface. 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 meets charging requirements. When the second charging interface is coupled to the first charging interface, the second charging interface applies the voltage of the third pulsating waveform to the battery, in which the voltage of the pulsating waveform output from the power adapter can be directly applied to the battery.