Along with the increasingly prominent issues aroused via Internet，online opinions are more and more concerned by researchers．This paper attempts to study the evolution of online opinions from two aspects，hot degree and BBS content，respectively．The original posts in Tiaya Zatan board，which is a primary board on social topics in Tianya Club，are collected as a source of online public opinion． By comparisons of a series of topic models，the dynamic topic model ( DTM) is chosen．After mining the dynamic topics，the evolutions of public opinions toward hot events are analyzed under a micro perspective of changing words，so as to outline the process of those events．A method is proposed to compute the hot degree of the topic，and then the hot de_x005fgree of all topics on the Tianya Zatan board in 2012 are computed and visualized，from which three rules of the topics are summarized under a macro perspective．