In recent years China has seen public crises take place very frequently. Public crises can cause panic to the public and severely affect people's study, work and living. Especially in an Internet environment, the openness, fastness, its great capacity to transmit and gather information of the Internet is an epoch-making. As a part of group behaviors in a public crisis, group network behavior is more and more pointing up the development trend of a crisis. Facts are enumerated and analyses are made in this paper, which aims to give a deep analysis of the causes, the development and the law of evolution of group network behavior using the examples of public crisis cases and relevant data. It tries to identify the obstacles to the use of Internet information by the public in a public crisis. Through the analysis of the influence and the role of these factors in a public crisis, this paper attemps to propose measures for controling and guiding group network behaviors in a public crisis.