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, 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 coupled sampling performed by the sampling unit at an auxiliary winding, such that a third voltage outputted by the second rectifier meets a charging requirement. The terminal includes a battery. When the power adapter is coupled to the terminal, the third voltage is applied to the battery.