Currently, Bitcoin are most popular ways to implement electronic cash transactions. As Bitcoin is one of the proof-of-work (PoW) blockchain implementations. This transaction mode also explored and applied in the electric power trading. Although the security of blockchain has been thoroughly analyzed, the security of forked PoW blockchains has not been carefully investigated. This paper builds a novel and quantifiable model of power trading systems based on blockchain technology. According to the behavior of forked blockchain, the analysis and model are proposed by Markov decision process (MDP). An MDP-like security analytical model for PoW blockchain by adding sidechains is established. In addition, a blockchain-based power trading simulator is designed. Through the simulation experiment, the influence of block generation interval and block size on the blockchain in the electric power trading system is analyzed.