Online public opinion is increasingly becoming a significant factor affecting the stability of the internet and society, particularly as the frequency of online public opinion crises has risen in recent years. Enhancing the capability for early warning of online public opinion crises is urgent. The most effective approach is to identify potential crises in their early stages and implement corresponding management measures. This study establishes a preliminary indicator system for online public opinion early warning, based on the principles of indicator system construction and the characteristics and evolution patterns of online public opinion. Subsequently, data-driven methodologies were employed to collect and preprocess public opinion indicator data. Utilizing grey relational analysis and the K-Means clustering algorithm, we classified online public opinion events into three levels: slight, warning, and severe. Furthermore, we constructed an online topic evolution model using the online Hierarchical Dirichlet Process model to analyze the thematic changes of online public opinion events across different warning levels. Finally, we developed an online public opinion early warning model using a Backpropagation (BP) neural network. The test results of early warning samples show that the model achieves high accuracy. Thus, in practical early warning applications, the BP neural network can be effectively utilized for predicting online public opinion events.