With the deepening of the reformation of power system and the massive access of distributed energy, the business model of electric power spot trading is being investigated. It is urgent to put forward a new trading mechanism to adapt to the new business model. Firstly, a decentralized power transaction mechanism is designed based on the idea of decentralization, which is suitable for the intraday spot market. It encourages prosumers to initiate real-time peer-to-peer trading demand according to their own actual demand of power generation capacity or electricity consumption. Prosumers can trade directly with others nearby. Then, the transaction rules are formulated in the form of smart contracts by applying the blockchain technology to define the rights and obligations of prosumers definitely. Finally, a distributed power trading platform with two interoperable layers is built by combining smart meter and smart terminal, multi-information fusion communication supported automatic clearing and settlement of transactions. The proposed decentralized power transaction mechanism can realize peer-to-peer multilateral bidding transactions among prosumers. It encourages prosumers to declare their trading demands appropriately, so as to achieve efficient distribution and rational utilization of energy resources. This work is supported by National Natural Science Foundation of China (No. 51777126).