Under the new situation, the network public opinion of sudden major epidemic not only brings severe challenges to epidemic prevention and control, but also brings great pressure to maintain the national ideological security. It is of great significance to clarify the generation logic of network public opinion, grasp the evolution characteristics of network public opinion, and do well in network public opinion guidance and rumor governance. In the modern society of network communication and practical life style, from the perspective of realistic logic, practical logic and theoretical logic, the generation of network public opinion on sudden major epidemic is the result of interaction among network topics, network contact and network communication. Under the influence of various factors such as the subject, object, environmental and intermediary of network social interaction practice, the public opinion of the network of sudden major epidemic presents a nonlinear trend and is prone to evoke the “Butterfly Effect” causing disorder, and even leading to the trust crisis of “Tacitus trap”, which brings complex variables to epidemic prevention and control. We must establish and improve the governance mechanism of network public opinion, effectively control public opinion, and improve the level of epidemic prevention and control.