Since the introduction of Bitcoin, digital currency has entered into a new era, and the underlying Blockchain technology has gradually attracted the attention of researchers. As the core of Blockchain technology, the consensus mechanism determines the security, scalability, and decentralization of Blockchain. This paper studies the existing consensus mechanisms from the perspectives of system model, consensus mechanism, incentive setting, and security attack. Firstly, network models, adversary models, and corruption models of consensus mechanisms are defined and classified. Based on different models, existing consensus mechanisms are divided into classic consensus and Blockchain consensus. Secondly, for the classical consensus, PBFT, Paxos, and the latest improvements are briefly introduced. In addition, a concrete algorithm, advantages, and disadvantages are given. Thirdly, Blockchain consensus is divided into permissioned consensus and permissionless consensus according to different application scenarios. This paper further divides the permissionless consensus into proof-of-work based consensus, proof-of-stake based consensus, single-committee based hybrid consensus, multiple-committee based consensus, and other consensus. For each type of consensus mechanism, the basic procedure is given, and typical solutions are analyzed in depth. The advantages and disadvantages, transaction throughput, and the possible attack methods of the consensus mechanism are given. Finally, this paper points out some research hotspots and development directions of Blockchain consensus mechanisms in terms of security, capacity expansion, startup, and incentive.