<p>Syntactic complexity refers to the degree of variety, sophistication, and elaboration of the syntactic forms used in language production. Although the syntactic complexity of academic writing has been widely investigated, little attention has been given to the diachronic changes of syntactic complexity in research article abstracts (RAAs) across disciplines. This study, therefore, compiled a corpus of 1,890 published RAAs from natural sciences, social sciences and humanities with a year-by-year sampling from 2000 to 2020, and examined the diachronic trends of RAA syntactic complexity in general as well as the disciplinary variations. Our findings show that RAA syntactic complexity has in general remained stable, possibly due to the interplay between the informational and promotional goals of RAAs. Regarding disciplinary variations, different disciplines revealed diverse or even contrastive changing patterns, which could be accounted for with the theory of disciplinary identities that bridges syntactic complexity and communication demand of RAAs. To be specific, natural sciences have increased in the length of sentences and T-units, the use of complex T-units, dependent clauses, coordinate phrases, complex nominals and verb phrases, probably attributed to the need of packaging complex knowledge and describing duplicable experiment details; social sciences have increased in the length of clauses and the use of complex nominals, but decreased in the amount of clauses and dependent clauses, which may be shaped by a trade-off between scientificity and readability; and humanities featured shorter clauses, fewer coordinate phrases and complex nominals, but more clauses, subordinations and verb phrases, potentially alienating their syntactic structures as a means of upholding disciplinary legitimacy. This study advances our understanding of the nature of RAA syntactic features, offering valuable insights for academic writing instructors, researchers, and scientometric analysts.</p>