Discourse markers have multiple pragmatic functions and have significant influences on classroom organization and teaching. This study aims to analyze the pragmatic functions of discourse marker so in a self-built corpus of Chinese middle school English classroom discourse. The corpus analysis shows that discourse marker so has various pragmatic functions, including marking results and inferences, managing topics, summarizing, rewording and giving examples at the textual level. Besides, at the interactional level, so functions as a marker of speech acts such as questions, requests, opinions, elicitations and confirmations. The research shows that discourse marker so can make classroom discourse more closely linked, promote students’ understanding of teacher’s discourse and create a harmonious classroom atmosphere. The findings of this research might have important implications for English teaching and teachers’ professional development in middle schools.