In order to avoid the negative influence of uncontrollability of charging and discharging behavior of electric vehicles on load curve of distribution system, first, from the point of view of energy balance, the charging/discharging energy is calculated by the energy consumption of EV driving distance. Then under the influence of electric power energy price, the EV charging/discharging behavior is determined by minimizing the charging cost. Then, the Distribution System Operator (DSO) dispatches the EV charging/discharging energy to flatten the load curve. Dynamic programming is used to optimize EV charging/discharging power. The developed method is tested in the PG&E 69-bus distribution system. Load shifting level is evaluated in the different energy price environment. The simulation results show that the charging and discharging strategy can effectively shift the peak load under the influence of electricity price. This work is supported by National Natural Science Foundation of China (No. U1766201 and No. 151407052).