An adapter, a charging control method and charging system are disclosed. The adapter includes a primary unit, a power adjustment unit and a secondary unit. The primary unit converts an input alternating current into a first current with a first pulsating waveform. The adapter is capable of operating in a constant current mode. A peak value of the first current is greater than a current value corresponding to a current limiting point of the constant current mode. The power adjustment unit samples current outputted by the adapter to obtain a current sampling value, modules the first current according to the current sampling value. The secondary unit converts current coupled from the primary unit to the secondary unit into the current. The current outputted by the adapter is a second current with a second pulsating waveform. A peak value of the second current is equal to the current value corresponding to the current limiting.