Preservice teachers' (PSTs) epistemic agency during their practicum experiences is not yet fully understood. In this study, we collected data in the form of in-depth interviews, reflective journals, and lesson plans from two Chinese language PSTs, and examined their epistemic agency during their practicums in Macau from a cultural-historical activity theory perspective. First, this study offers an analytical model that revealed the dynamics of epistemic agency development during PSTs' practicums. Second, the analysis showed how PSTs' social situations and prior learning experience recursively shaped their epistemic agency. The study raises important implications for teacher education, highlighting the importance of supporting PSTs' epistemic agency by providing epistemic resources.