Conjunction "shiyi" is a common causal marker in pre-Qin Chinese, and there are four views about its origin structure, namely, the "preposition + object" structure, the "conjunction + conjunction" structure, the "object + verb" structure, and the "pronoun + conjunction" structure. This study aims to determine and explain the lexeme(s), the lexical item(s), and the semantic evolution path of the conjunction "shiyi" in terms of the theory of grammaticalization as expansion. It is found that the conjunction "shiyi" occupies one single lexeme, but two lexical items (or sememes), one "descriptive resultative conjunction", the other "inferential resultative conjunction". The latter sememe is the product of the former one’s further grammaticalization. The conclusion supports the mainstream view that conjunction "shiyi" originates from the inverted "object + preposition" structure, but opposes the views of "conjunction + conjunction" structure and the "pronoun +conjunction" structure. The possibility could not be ruled out that conjunction "shihi" originates from the lexicalization of pronoun "shi" and preposition "yi" in declarative sentence "shi+yi+NP+VP+ye".