This paper reports on the implementation of a 6-month collaborative teacher education project (CTEP) in China, designed to help teachers adopt CLIL in response to new primary school curriculum requirements. A multi-site case study was conducted to track two focal teachers' changes in CLIL implementation and its sustainability. Adopting ecological theory, the study investigated how teachers' interactions with university-based, district-based, and school-based teacher educators in the project contributed to their professional development. Our research focused on an innovative dimension of the project, namely, close collaboration among university researchers, teacher educators, and primary teachers who all contributed to the CTEP ecosystem. Data collection involved semi-structured interviews at the pre-stage, while-stage, post-stage, and delayed post-stages of the project, classroom observation notes, lesson study minutes, field notes, informal exchanges, and project documents. Our data analysis revealed that although both teachers had a positive attitude towards CLIL, they exhibited different trajectories of changes in their pedagogical practices. The teachers' interactions with teacher educators within and across the four sub-contexts in the CTEP ecosystem played a significant role in their professional development. Notably, the school-based teacher educators contributed by establishing a collaborative teaching study group (TSG) that ensured sustainable professional development for the teachers. The findings of this study have important implications for educational policymaking and for designing and implementing collaborative teacher education programmes that offer an alternative to traditional top-down modes of language teacher professional development.