As the core technology of blockchain, consensus protocol has received great attention from academy and industry in recent years, and made a series of research achievements. Existing surveys on consensus protocols generally regard the consensus protocol as a whole, and do not decouple and compare its main components. In this survey, the consensus protocol is divided into two main components, i.e., blocker election and main chain consensus, and their analysis and comparison are conducted correspondingly. In the blocker election component, two mechanisms are mainly discussed, i.e., proof of work and proof of stake. For each mechanism, the encountered problems are analyzed and their corresponding solutions are compared with each other. In the main chain consensus component, its security goal is summarized and security comparison is conducted for probabilistic consensus and deterministic consensus. Through the comprehensive review of the state-of-the-art consensus protocol in blockchain, the developing status, developing trend and some important research directions are finally summarized for the consensus protocol.