Under the new media environment, the information dissemination speed is fast, the action subject is diverse, which make the network public opinion governance more difficult and complex. It is of great significance to pay attention to the communication process and nodes of public opinion events for correctly coping with the network public opinion and reducing the impact of the network public opinion crisis. Taking the incident of "49 middle school’ student falling from a building" in Chengdu as an example, this paper uses the web crawler technology to obtain the blog posts published by netizens around the incident on Sina Weibo as the research data, analyzes the theme distribution of online public opinion through LDA theme model, and explores the overall situation of public opinion triggered by the incident combined with the spiral theory of silence to focus on the network public opinion discussion and derivative public opinion caused by this event. It is found that the Internet public opinion mainly focuses on five aspects: the introduction of the key information of the event, the query of the school, the query of the relevant institutions, the empathy for parents and the evolution of public attitude after the news reversal.