<p>Research on academic writing has reported divergent diachronic trends in sub-indices of syntactic complexity, leaving the overall diachronic trajectory inconclusive. Moreover, limited attention has been paid to meaning-related complexity (i.e., semantic density) and to its interaction with syntactic complexity. Drawing on a self-built corpus of 3000 research article abstracts from Applied Linguistics, Sociology, Biology, and Electrical Engineering between 1975 and 2024, this study investigated the diachronic trajectories of syntactic complexity and semantic density and their interaction. Results revealed an increase in syntactic complexity across all four disciplines, characterized by greater normalized dependency distance. This indicates a growing syntactic elaboration that places a higher cognitive demand on sentence processing. Conversely, semantic density decreased in Biology and Electrical Engineering while remaining stable in Applied Linguistics and Sociology. This indicates a reduced degree of meaning condensation in hard sciences. Notably, Biology and Electrical Engineering exhibited higher levels of both features and greater fluctuations across adjacent time periods. Furthermore, a negative correlation between syntactic complexity and semantic density was observed, suggesting a trade-off between structural elaboration and semantic compression in academic writing. These findings are interpreted by evolving disciplinary conventions, rhetorical demands, cognitive load, and broader sociocultural changes. Overall, this study contributes to a more nuanced understanding of the diachronic trajectories and interaction of syntactic complexity and semantic density in academic writing.</p>