This study aimed to examine the role and process of reflective assessment supported by the Knowledge Connections Analyzer in helping low academic achievers to develop epistemic agency in knowledge building. The participants were 33 ninth-grade low achievers from a visual arts course in Hong Kong. A comparison class of 33 students, taught by the same teacher and studying the same topics in a regular knowledge-building environment, also participated. Qualitative tracing of students' online discourse showed that reflective assessment can help low achievers develop high-level epistemic agency. Qualitative analysis of the students' prompt sheets revealed that reflective assessment encouraged low achievers to set knowledge-building goals, collectively and continuously analyse and reflect on their inquiry and ideas, and generate actions to address identified gaps, thus helping them engage in high-level epistemic agency. The study results have important implications for designing technology-rich environments that support learners and offer insights into how teachers can help learners develop epistemic agency.