This paper based on the propagation characteristics of the new media，and used Lasswell’s “5W” mode as the blueprint. From the relationship between disseminator，audience，content，channel and effect factors and their propagation as the clue，selects the “freeloading by the son of the director of the Education Bureau of zhangzi County in Shanxi” event give the example of a case analysis. It is used the content analysis to sort out the propagation process on the whole event. According to the logical progressive nature of the event propagation process，the conversion between different media，the impact on public opinion. It is used the dissemination of key time node stratified identification to built the event-node database so that to explore the network structure，the propagation path of the node position and the relationship between information and communication speed and other issues in the unexpected public incidents. The study showed that the dissemination and diffusion of unexpected public incidents is complex and dynamic，the time node in this development of the event is the key point that spread between the groups，the network structure and the time node position determines the degree of influence of the members，the key points can quickly penetrate into groups. In the case of unexpected public incident，it can be embedded boot time nodes by changing the agenda setting or seizing the communication of the key path to reduce the spread of rumors and fear.