Abstract: research explores the semantic changes of English semi-auxiliary construction "be supposed to" via EEBO and COHA corpus data.The results show that apart from the semantic change of the lexical verb suppose itself in active voice, its epistemic meaning may also be influenced by "be supposed that"as is predicted by"the Great Complement Shift", as well as the epistemic "be thought to/that/Φ"construction by means of paradigmatic analogy.Meanwhile, its frequency increase is reinforced by the deontic schematic construction "BE pp.TO".As to the deontic "be supposed to", on the one hand, this modal meaning may have derived from the volitive "have/had supposed to" in active voice; on the other hand, it may also be indirectly influenced by other semantically and morphosyntactically similar constructions such as "be obliged to".