This paper presents a fairly exhaustive study of the historical evolution of "shi" and reasons of this historical evolution. In this paper the references functions in "Wei Mo Jie Suo Shuo Jing" (維摩詰所說經) of different "shi"(是) are discussed and the syntactical patterns and expressing function of "shi" are studied from the view point of combining diachronic changes with synchronic variations. Furthermore, the cause and mechanisms of the grammaticalization, as well as the connection between historical evolution and modern coherent functions, of "shi" are explored. Copula- "shi" is founded on the typical Archaic Chinese sentence pattern "NP+者,O是+NP+也." Due to frequent and repeated employment of this structure, its semantic meaning is generalized, which in turn results in the weakening of its reference nature and vanishing of its original meaning "this," and the pattern of "NP+者,O是+NP+也" tramsforms from the structure of "topic [pause], subject" to the structure of "subject+shi+verb phrase," in no longer designates any reference information.