The invention provides a charging method and a direct current charging system, which is applied to the technical field of power supply. The method comprises the steps of taking a charging interface which is connected with a charging object in a plurality of charging interfaces of the direct current charging system as a target charging interface, acquiring required charging power of the charging object connected with the target charging interface in a constant-power charging mode, then combining conversion modules in the direct current charging system according to the required charging power toobtain a plurality of conversion module groups, further determining the conversion module group with the minimum charging loss value in the plurality of conversion module groups to obtain a target conversion module group, and finally controlling the conversion modules in the target conversion module group to charge the charging object. According to the method, when the charging system charges thecharging object each time, the conversion module group with the minimum charging loss value is selected to charge the charging object, so that the efficiency of the direct-current charging system ina constant-power charging mode stage can be effectively improved, the electric energy loss in the charging process is reduced, and the charging cost is reduced.