Since the discussion regarding the structure of “Pre+NP+X” in Chinese grammar has begun, many scholars agreed that “Laishuo(来说)”, “Kanlai(看来)”, “Qijian(起见)”, “Qi(起)”, “Weizhi(为止)” etc. are grammaticalized elements. This study mainly studies the grammaticalization preocess and the mechanism of “Qi(起)”. Through the diachronic analysis, we obtained the following results. First, in the Warring States period，“Qi(起)” was used as a verb to indicate the meaning of “emerging, rising” and appeared in the structure of “Pre+ NP+Qi(起)verb”. Second, it became grammaticalized as a quasi-auxiliary word(准助词) when it began to appear in the structure of “Pre+NP+Qi(起)+VP” in the Song Dynasty. In such cases, the main predicate in the sentence is moved from “Qi (起)” to the predicate of the subsequent component, and “Qi(起)” cannot function as the verb “emerging, rising” so it can be seen that it has already been grammaticalized. Third, in the Qing Dynasty, as the number of appearance of “Pre+NP+Qi (起)” structure has increased significantly, the number of prepositions combined has increased, and the NP composition has become more diverse, showing the phenomenon of mature grammar as a whole. Forth, in the early 20th century, the verb use of “Qi(起)” has almost disappeared in the structure of “Pre+NP+Qi(起)”，and all of it has been used as a quasi-auxiliary words. “Pre+NP+Qi(起)” has been developed by special prepositional phrase that represent the meaning of time.