Internet＋government affairs big data has the characteristics of cross-domain, multi-protocol and difficult convergence. In the process of big data collection and aggregation, there is a need for a variety of protocol conversions, which require a gateway to realize a unified protocol adaptation conversion and provide the data support for multi-source heterogeneous data aggregation and data fusion. Traditional protocol conversion methods are usually designed for specific protocol conversion requirements, and their scalability is poor, so they are not suitable for the situation where there are multiple protocol conversion requirements. By studying and analyzing the structure of protocol messages and the characteristics of protocol conversion, this paper proposes a method to construct the knowledge graph of the protocol conversion. By constructing the schema layer and instance layer of the graph, a knowledge graph of protocol conversion containing the structure of protocol messages and the mapping relationship between message fields is established. On this basis, a protocol conversion method based on the knowledge graph is proposed to realize the packet conversion between different protocols. The effectiveness of the proposed method is verified by an application example of protocol conversion and a comparison experiment with the existing protocol conversion methods.