In order to solve the problem that the number of charging piles is insufficient and the platforms of each charging pile operator are independent, a strategy of sharing platform of EV charging piles based on block chain is proposed in this paper. Firstly, the feasibility of building platform with block chain is anaylzed, then the electric vehicle charging pile sharing platform is established on the basis of block chain and the workflow of the trading platform and user utility function is designed. Secondly, a three-layer charging pile strategy is constructed to find the optimal charging pile for EV users by introducing the particle swarm optimization (PSO) algorithm. Finally, the simulation results show that the platform security can be improved by utilzing block chain, and the user utility function is improved at the same time. The three-layer charging pile strategy can find the optimal charging pile in terms of different needs of users. With the proposed stratrgy, the charging cost of users is reduced, the utilization ratio of charging post is increased and the collective charging number of EVs is reduced at night.