Few studies have explored primary school teachers’ classroom-based online assessment practices and underlying reasons. To fill this research gap, this study interviewed 48 Hong Kong primary school teachers to understand their online assessment practices and influencing factors when they were obliged to use it in their daily instruction under the influence of the COVID-19 pandemic. The findings revealed that the participants tended to use online tests/exercises for formative purposes instead of summative purposes. In addition, they tried online alternative assessment tasks, such as video or audio recordings, peer assessment and projects, and gave online feedback to students but less frequently than online tests/exercises. The school examination culture and the participants’ perceived limited control over online test fairness may have restricted their summative use of online tests/exercises. Meanwhile, the participants’ perceived positive usage norms, along with their favourable attitudes towards and confidence in using online tests/exercises probably enhanced their formative use of them. In addition, the participants’ perceived neutral usage norms and limited external control of online alternative assessment tasks and feedback seemed to impede their use of them in classrooms.