This paper uses the classical propagation model SIR model to analyze the topic propagation mechanism and establishes a model to verify it through simulation experiments. In addition, this paper constructs a multi-topic propagation model which is can be used to monitor the topics' trend. Through the simulation results we can see that the research is significant as if the topic occurs at longer intervals, the smaller the dissemination and spread of the first topic, the greater the dissemination and diffusion of the second topic. Moreover, if the relevance of the topic is greater., the dissemination and diffusion of the two topics will become larger simultaneously. This method is useful to detect the topics in other fields such as social network and newspapers., etc.