The present disclosure discloses a charging system and a charging method for a terminal, and a power adapter. The charging system includes a power adapter and a terminal. The power adapter includes a first rectification unit, a transformer, a second rectification unit, a sampling unit, and a modulating control unit. The modulating control unit modulates a voltage with a first pulsating waveform according to a voltage sampling value sampled by the sampling unit, such that a voltage with a third pulsating waveform outputted by the second rectification unit meets a charging requirement. The terminal includes a second charging interface and a battery. The second charging interface is coupled to the battery. When the second charging interface is coupled to the first charging interface, the second charging interface applies the voltage with the third pulsating waveform to the battery, such that the voltage with the pulsating waveform outputted by the power adapter directly applies to the battery, thus realizing miniaturization and low cost of the power adapter, and improving a service life of the battery.