With the continuous development of digital currency, the blockchain technology has attracted more and more attention, and the research on its key technology, consensus mechanism, is particularly important. The application of blockchain technology in the Internet of Things(IoT) is one of the hot issues. Consensus mechanism is one of the core technologies of blockchain, which has an important impact on IoT in terms of decentralization degree, transaction processing speed, transaction confirmation delay, security, and scalability.Firstly, the architecture characteristics of IoT and the lightweight problem caused by resource limitation were described, the problems faced in the implementation of the blockchain in IoT were briefly summarized, and the demands of blockchain in IoT were analyzed by combining the operation flow of bitcoin. Secondly, the consensus mechanisms were divided into proof class, Byzantine class and Directed Acyclic Graph(DAG) class, and the working principles of these various classes of consensus mechanisms were studied, their adaptabilities to IoT were analyzed in terms of communication complexity, their advantages and disadvantages were summarized, and the combination architectures of the existing consensus mechanisms and IoT were investigated and analyzed. Finally, the problems of IoT, such as high operating cost, poor scalability and security risks were deeply studied, the analysis results show that the Internet of Things Application(IOTA) and Byteball consensus mechanisms based on DAG technology have the advantages of fast transaction processing speed, good scalability and strong security in the case of having a large number of transactions, and they are the development directions of blockchain consensus mechanism in the field of IoT in the future.