The fantasy theme analysis based on the theory of symbolic convergence can effectively explain the common imagination among groups and reveal the suppressed meaning in the discourse. In the field of communication, many scholars use it to analyze social groups with obvious characteristics, or to focus on the online texts with a certain topic. Based on the analysis of fantasy themes, combined with the discussion texts of the public from the Weibo after the spread of Omicron in China, this research aims to outline the evolution of the public's discourse on the COVID-19, and to analyze the public discussion and consensus and the epidemic imagination.

This study believes that although the public discussion aggregated from Weibo may reduce health risks to a certain extent, may not be able to form unity on some topic discussions. The divergences may will push society to high risk position，affect epidemic prevention and control work. Based on this assumption, Based on this assumption, this study selects the top 10 topic texts from the related topics of Weibo Omicron from December 9th, 2021 to the present. According to the representativeness and popularity, this study finally selected 11,100 samples, extracted fantasy themes and analyzed their possibility of being discussed, and summarized the fantasy types, constructed a rhetorical vision, and further explored the risk factors behind the theme discussion, try to submit proposals about balancing conflicts and improving dialogue to reduce the risks.

The research shows that uncertainty of risk rise the possibilities for discussions among rhetoric members and present diversification of fantasy themes. Around the judgment of the epidemic risk, this study mainly formed two types of illusions: one is that the risk is reduced, but the prevention and control is excessive; the other is worse, and safety is the priority. Discourse differences not only reflect the differences in public imagination of risks, but also remind the authorities to speak out in time in interpreting policies and predicting risks. Government need to intervene in public discussions at the right time and try to build a rational public imagination by building a scientific discourse system of epidemic prevention.