Based on scale free network, the public opinion evolution model is constructed, in which the public opinion changes with topology (the relationship and the influence between the individual). The model features that the network structure constantly changes in the process of evolution with evolution parameter φ0. It is found that the evolution of the public opinion not only is controlled by the topological structure, but also induces the change of the topological structure: the network structure is no longer the initial scale-free network,but it has a Poisson distribution. With the time evolution in system, there is obvious convergence effect of the public opinions. The dozens of opinions in initial state evolve with time, but most of them perish and only a few of them can survive and develop for a long time. The evolution trend is coincident with that of the public opinions, views and beliefs in society.