Floods have become the most prevalent natural disaster in the world. Understanding the emotional impact of floods on people helps reduce negative mass incidents and contributes to the implementation of disaster prevention and mitigation. This study collected online discussions related to the great flood in the middle and lower reaches of the Yangtze River basin in China in 2020. Then, we obtained the sentiments of people and the deep thoughts behind their negative sentiment through the dictionary sentiment analysis and the LDA topic modeling, counting them according to the regions. The overall sentiment of Internet users during this flood was neutral, and the posts showing negative sentiment were mostly from the affected areas. People in the affected areas were highly concerned about personal safety and property, whose negative emotions were directed towards the flood prevention work and the relocation of affected people.