A charging method is provided. The method includes: receiving a direction setting signal, and determining a charging direction according to the direction setting signal; determining a charging operation as charging a slave device with a master device when the charging direction is a first charging direction; determining the charging operation as charging the master device with the slave device when the charging direction is a second charging direction opposite to the first charging direction; and sending to a power management module of the master device and to a power management module of the slave device a charging instruction according to the charging operation determined. Accordingly, two smart devices connected through a connecting line may be charged by each other according to a user-defined charging direction.