A charging system and charging method for a terminal (2), and a power adapter (1) and a switch power source. The switch power source comprises a first rectification unit (101), a switch unit (102), a transformer (103), a second rectification unit (104), a sampling unit (106) and a control unit (107), wherein the control unit (107) outputs a control signal to the switch unit (102) and adjusts a voltage of a first pulse waveform according to a voltage sampling value and/or a current sampling value sampled by the sampling unit (106) to obtain a primary sampling voltage, and adjusts a duty ratio of the control signal according to the primary sampling voltage, the voltage sampling value and/or the current sampling value, so that a voltage of a third pulse waveform satisfies a charging requirement. The switch power source can make an output voltage and current waveform change along with an input voltage and current waveform, so that a good power factor can be obtained. Moreover, when the switch power source is applied to the power adapter (1), a voltage of an output pulse waveform can be directly loaded to a battery of the terminal (2), so that miniaturization and low costs of the power adapter (1) can be realized.