This study examined how Chinese college teachers developed their Technological Pedagogical Content Knowledge (TPACK) by situating the learning process in three interactive activity systems during teaching English as foreign language (EFL) online in China. Data were collected from teachers’ diaries, in-depth interviews, and various records of online teaching. Findings indicated that teachers employed cultural artifacts, social relations, and beliefs and concepts to cope with difficulties that confronted their online teaching. They also performed sideways moves to new activities that provided systemic implications for the previous form of activity. Hence, the teachers constructed their TPACK through bi-directional learning within and between an online teaching activity system and its two interactive systems. This study highlighted the situativeness and distribution of TPACK development, and assumes significance in sustainable teacher development and further integration of information and communication technology (ICT) in English language teaching.