Social reading is an interactive reading activity that activates readers to read and discuss through social annotation, facilitates the expression of diverse ideas, and promotes collaborative inquiry and knowledge building. Social reading is a common activity in higher education. However, discussions on how social annotation can be combined with role assignment strategies and thus influence synchronous social reading are still insufficient. Therefore, this study designed a social reading activity for postgraduate students based on a role assignment strategy. Using social network analysis (SNA, a method capable of recognizing structural patterns and key participants of interpersonal networks in social reading activities) and epistemic network analysis (ENA, a method capable of quantitatively and qualitatively analyzing the degree of learners' textual knowledge acquisition in social reading activities), this study analyzed social annotation data from 12 rounds of collaborative social reading activities to explore how social reading activities based on role assignment strategies affect the degree of social interaction and knowledge construction of learners. The results showed that social reading activities based on role assignment strategies had significant positive effects on both social interaction and knowledge construction of learners. In addition, it was found that the implementation of the activity had a more obvious effect on learners with lower levels of prior knowledge as opposed to learners with higher levels of prior knowledge. This study identifies patterns in social reading courses about stages of learning and how it works for different learners, which has practical implications for online collaborative learning in higher education.