In recent years, the rapid development of social networks has significantly changed the styles of people's work and life.The way people acquiring knowledge shows a clear trend of the network.When people acquire information through the Internet, it also leaves personal traces.Taking the high cost of obtaining personal information in reality into account, it's a good idea to catch people's leaving traces in the network and study its "mapping" in the network society.User portrait as a virtual representative of real users is based on a series of real data on the user model.Based on dynamic modeling of user portrait, the user is defined as the basic attributes, social attributes, interests, and abilities by digging the "ZhiHu" site.Then we make an empirical analysis of the 1303 users who follow the topic of PM 2.5.