This study explores how teachers construct and enact agency in professional learning from an activity theory perspective. Through a qualitative case study of three language teachers' experiences within a professional development (PD) program focusing on China's Standards of English Language Ability (CSE), this research identifies agency construction as a triadic connection-building process among teachers (subjects), the CSE learning target (tool) and teachers' professional development goals (objects). The findings suggest that PD programs can be designed to assist teachers in building these connections by understanding their broader professional needs, guiding them to identify specific learning objectives relevant to their professional practices and supporting their application of the CSE to achieve these objectives. This study contributes to the literature on teacher professional learning by offering a nuanced understanding of teacher agency construction and proposing an agency-oriented approach to PD program design and implementation in facilitating teachers' engagement with new tools like the CSE to support sustainable professional growth.