Multilingual identity construction is a diachronic, non-linear and constantly evolving process which is influenced by a complex range of contextual and temporal factors. Yet the temporal dimension seldom receives explicit empirical attention and remains under-theorised. Meanwhile, the dynamics of multilingual identity development are rarely explored at the cohort (i.e., whole-class) level. Underpinned by complex dynamic systems theory, this study tracks the evolution of multilingual identity positions in a class of Japanese-learning public high school students in China using longitudinal Q methodology. Three rounds of Q sorting and Q-based interviews were administered over the course of one academic year. The findings testified the enrichment and concretisation of students' multilingual identity with the accumulation of their Japanese learning experience. A range of sociocultural factors with changing saliencies impacted the students’ identity development, giving rise to group-level patterns of convergences and divergences at different timepoints. We argue that the temporal axis should be explicitly conceptualised in multilingual identity research. A temporal model of multilingual identity development is proposed to better contextualise and construe the continuity and shifts of multilingual identity at both individual and group levels.