It is widely believed that teachers’ knowledge of students’ thinking has a significant impact on teachers’ teaching and students’ learning. However, there is far less research on how teachers acquire their knowledge of students’ thinking before, during, and after lessons. This study is designed to compare the differences between expert and nonexpert mathematics teachers on their behaviors and perceptions related to understanding students’ mathematical thinking. Based on 554 Chinese elementary mathematics teachers’ responses to a survey, the study found that teachers took actions to understand students’ thinking more often when students were learning new topics or encountering difficulties, and they were more likely to do so before lessons than during or after lessons. The comparison revealed that significantly more expert elementary mathematics teachers attempted to understand students’ thinking from a variety of perspectives before making the necessary adjustments to their predetermined teaching plans than did nonexpert teachers. Significantly more expert teachers also relied on their own teaching experiences to understand students’ thinking. In contrast, significantly more nonexpert teachers claimed that they did not rely on prior teaching experiences because they “did not know how to.”