With the development of novel power system, the virtual power plant (VPP) which can aggregate and manage the distributed energy resources effectively has developed fast. Under the background of power market, the VPP acts an individual stakeholder in energy trading and network operation. This paper proposes a Block Chain based P2P energy trading strategy in power retail market for MVPPs. The operation framework for MVPPs is established with employment of blockchain, which is in a distributed structure to reduce information communication burden, low credibility of participants and privacy protection. The energy trading model is established to maximize the commercial profits with consideration of physical constraints. Numerical simulations are conducted on a four VPPs test system to verify the validity and reliability of the proposed strategy.