Large-scale electric vehicles charging in disorder will overlap with the basic load of power system, increase the peak valley difference of the system, cause the heavy overload of distribution transformer and the increase of power system network loss, thus threatening the safe operation of power grid. In order to realize the orderly charging of electric vehicles, based on the load fluctuation difference as the network loss analysis index, a real-time optimal constant power orderly charging model of electric vehicles is proposed. According to the conventional load forecasting curve and the basic data of electric vehicle charging, the real-time optimal constant power is solved. On the premise of meeting the maximum capacity of distribution transformer and other constraints, the orderly charging scheme of electric vehicles is formed with the goal of minimizing the load fluctuation difference of power system. Finally, the simulation results of IEEE 33 bus system with MATLAB platform show that the proposed orderly charging strategy can more effectively achieve the goal of "peak shifting and valley filling", achieve the goal of stabilizing load fluctuation and reducing system network loss, thus verifying the effectiveness of the strategy.