As the COVID-19 pandemic began to spread all over the world, many educational institutions have shifted to a full-time online teaching mode. Although online teaching has been widely explored, the unprecedented initiatives of mass-scale full-time online education at the secondary school level are yet to be unravelled. By using a qualitative approach and drawing on the conceptualisation of learning engagement and Community of Inquire model as conceptual frameworks, this study explored how secondary school students in China engaged with online education during the COVID-19 pandemic and what factors influenced their sustainable online learning engagement. This research examined the perspectives of twenty-four students and five teachers through semi-structured interviews and observations of online classes. Findings indicate that the students’ online learning engagement involved three inter-related categories: emotional, cognitive and behavioural engagement. Contextual factors influencing the sustainability of students’ online learning engagement were identified by the participants, including teacher presence, parental involvement, and a supportive learning environment/community. The findings in this paper have implications for teacher development, family support and establishment of e-teaching platforms in emergency remote teaching for young students. Finally, the study puts forward best practices for the sustainable development of the emergency remote teaching in the future public crises.