A method and a device for charging an electric vehicle in a power system are provided. The method includes: obtaining a plurality of electric vehicles connected to the power system at a dispatching time, and obtaining a rated charging power and a charging requirement at the dispatching time; determining a charging period corresponding to the plurality of electric vehicles; determining a forecast period, and obtaining a charging requirement, a remaining charging energy capacity and a maximum charging power; establishing a charging model of the plurality of electric vehicles, 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 of each electric vehicle at each charging time in the charging period so as to charge each electric vehicle under the optimal charging power.