In this study, we integrated corpus technology in pre-service TESOL (Teaching English to Speakers of Other Languages) teachers' technological pedagogical content knowledge (TPACK) development in corpus technology, termed corpus-based language pedagogy (CBLP), and highlighted the collaborative effort for knowledge building among participants for TPACK development. The study examined the role of online collaboration in facilitating how pre-service TESOL teachers developed their CBLP (TPACK in corpus technology) using a learning-by-design approach, enacted it in co-designing lessons, revised their lessons and reflected on their collective knowledge building processes. Using a case study approach involving 33 participants, the study focused on both intra- and inter-group interactions to understand the pre-service teachers' learning dynamics/changes and unpack the interaction mechanism involved in online collaborative learning. To these ends, we collected data including CBLP group lesson designs, intra-group data (interviews) and inter-group peer critical comments and analysed them based on the two stages (comprehension and transformation) of Shulman's pedagogical reasoning. The results suggest that engaging in online collaboration, facilitated by the learning-by-design approach, is instrumental in enabling pre-service teachers to learn corpus technology and expand their repertoire of teaching strategies. Our findings imply that both intra- and inter-group collaboration modes are important to help pre-service TESOL teachers holistically develop TPACK for language teaching. Similar implications may be applied to other subject-specific TPACK training.