This paper discusses what kinds of topics are easier to become issues of public opinion and the formation mechanism.Through the empirical analysis of the posts of QiangQiang Tittle-tattle in a concentrated time,the results are the following:it is easier for topics which are spontaneous and unconventional to turn into hotly discussed issues online.Topics with resonance effect and spill-over effect that can satisfy some collective mind are apt to diffuse widely.Online public opinion leaders play a role as catalyst in the process.The formation mechanism of online public opinion includes four stages:the spread of personal opinions,circulation,opinion leaders intervention and the maturity of issues. <<