A large number of disordered plug in hybrid vehicles（PHEVs）are connected to the power grid

which will cause overheating and overload of the power grid transformer during peak hours

resulting in tripping or even large scale power outages.Therefore

the problem of coordinated charging of electric vehicles is a research hotspot and difficulty in the power grid. The problem of coordinated charging of plug in hybrid vehicles is firstly described as an optimization problem with constraints. Then a two layer optimal charging strategy is proposed to solve the optimization problem. In the first layer

the load curve of the low voltage trans formers（LVTs）of the grid is flattened and smoothed based on the demand side management；the second layer is based on the consistent iterative algorithm to make the overall charging of the plug inhybrid vehicle users. The cost is minimized while meeting the user’s charging needs. The proposed optimal charging strategy not only keeps the fluctuation of the power supply load curve of the power grid transformer to the minimum

but also realizes the minimum charging cost of each electric vehicle user and meets the charging demand of the user.