With a sample of 4,646 undergraduate students from four universities in China, this study examined the psychometric qualities of a newly-adapted measurement, namely, the Online Motivation and Engagement Scale for university and college students, and investigated the characteristics of and differences in Chinese students' motivation and engagement in emergent online learning during the COVID-19 pandemic. The results supported the reliability and construct validity of the measurement, and identified four types of learners with distinctive features of student engagement. The research revealed a complex picture with promising and worrying messages about student engagement in emergency online learning during the pandemic. These findings contribute to the conceptualisation and measurement of student engagement in online learning, and provide insights for improving online learning and teaching during and beyond the pandemic.