This study examined the trajectories of the multi-word constructions (MWCs) in 98 advanced second language (L2) learners during their first-year at an English-medium university in a non-English-speaking country, using linear mixed-effects modelling, over one academic year. In addition, this study traced the academic reading input that L2 learners received at university, and it was investigated whether the frequency and dispersion of the MWCs in the input corpus would predict the frequencies of MWCs in L2 writers' essays. The findings revealed variations in the frequencies of different functional and structural categories of MWCs over time. This study provides empirical evidence for the effects of both frequency and dispersion of MWCs in the input corpus on the frequency of MWCs in L2 writers' essays, underscoring the importance of both frequency and dispersion in learning MWCs and the reciprocity of academic reading and writing. The findings have significant implications for usage-based approaches to language learning, modelling MWCs in L2 academic writing, and L2 materials design for teaching academic writing.