[Objective] This paper summarizes the developments of early warning research for online public opinion. [Coverage] We searched the Web of Science and CNKI with related terms such as public opinion warning, online public opinion, and public opinion risks. A total of 52 articles representing the foundations of the disciplines and the development trends were selected for a comprehensive review. [Methods] We summarized the foundations of early warning studies from the perspective of online public opinion characteristics and risk evaluations. Then, we examined the status quo of current research on early warning for online public opinion. [Results] Currently, most research focuses on expert empowerment, machine learning, communication process, and sentiment analysis. All of them can accurately predict the risk level of online public opinion, which is significant to the online environment and social stability. [Limitations] More research is needed to review early warning technology. [Conclusions] The existing research does not provide universal concepts for online public opinion. The risk evaluation method needs to be improved. We should also establish authoritative and unified standards to compare the performance of different monitoring systems.