A method and a device for charging an electric vehicle in a power system are provided. The method includes: obtaining a first electric vehicle connected to the power system, and obtaining a rated charging power and a first charging requirement; determining a first charging period corresponding to the first electric vehicle; determining a forecast period, and obtaining a second electric vehicle to be connected to the power system; revising the first charging period to obtain a second charging period, and obtaining a second charging requirement and a maximum charging power; establishing a charging model, establishing a first constraint of the charging model, and establishing a second constraint of the charging model; and solving the charging model under the first constraint and the second constraint to obtain an optimal charging power so as to charge each first electric vehicle under the optimal charging power.