This study explored the development trajectory of shared epistemic agency in online collaborative learning through epistemic network analysis and lag sequential analysis. It was carried out in a postgraduate course with 14 in-service teachers. Drawing on the online discussion data from six sessions and the participants' academic scores, this study found a nonlinear development trajectory of learners' shared epistemic agency across the six sessions. The managerial dimension (e.g., regulative and relational actions) mediated the development of learners' shared epistemic agency. The analysis of different groups' mean networks and academic performance revealed a tentative relationship between them. Finally, the transition of shared epistemic agency actions in higher-achieving sessions and groups largely followed an upward sequential pattern. This study provides a graphical insight into how students learn in an online collaborative setting and can inform future pedagogical and technological designs of facilitating students' shared epistemic agency for the creation of collective knowledge.