At this stage, China society in conflict-prone period, the network group events developed rapidly. Network group events had a significant impact on society. The trend which the online group event evolved into the offline group incidents is becoming evident. In this paper, we used grounded theory to study the evolution and the development process of network group events under the micro-blog platform. We present an evolution mechanism model of network group event under micro-blog platform and find the formation mechanism of the network of public opinion, intrinsic motivation and the evolution of the network group events.