Peer assessment (PA) is a formative assessment tool that can effectively monitor the development process of knowledge construction. In comment-based PA, comments contain the evidence of how the assessors construct knowledge to conduct professional assessments, which initiates a research perspective to explore the dynamic knowledge construction of the assessors. Quantitative ethnography is both a method for the quantitative analysis of qualitative data and a technique for the network modelling of professional competencies, providing a new way of thinking about the analysis and evaluation of knowledge construction processes. In this paper, quantitative ethnography was used to mine the comments generated from comment-based PA activities to reveal the characteristics of student teachers’ knowledge construction and the developmental trajectories of knowledge structure at different learning stages. The experimental results show that the student teachers’ knowledge structures and knowledge levels evolve in the PA environment, and the cognitive network gradually tends to become more complex and balanced. The student teachers showed stage and gender differences in the level of knowledge progression during the learning process. The second PA was a turning point in knowledge progression. The knowledge structures of the male and female groups are biased towards different kinds of knowledge elements.