Flexibility of the part of speech is a very common and important linguistic phenomenon in ancient Chinese. The author aimed to describe and probe into it by investigating the flexible use of the nouns in Zuozhuan which is a large-scale literature in the Pre-Qin Dynasty. In Zuozhuan, there are 89 nouns which occasionally offend their routine grammatical characteristics and are flexibly used as verbs, adverbs, etc. for as many as 227 times. These nouns have the following characteristics: On the grammatical aspect,they have evident structural characteristics when they are flexibly used as other parts of speech occasionally. On the semantic aspect, there are inseparable relations between the meanings of the nouns before and after flexible use. They can be summarized into derivation, metonymy and metaphor. On the pragmatic aspect, the flexible use of nouns has special expressive functions, which are intensively embodied in styles, personal characteristics and contexts. By investigating the flexible use of the nouns in Zuozhuan,it can be concluded that "flexibility of the part of speech" is a special grammatical phenomenon that originated from and accommodated the characteristics of Chinese language. It is evidently different from the combination of the parts of speech, interchangeability of words and read. It is one of the important structural methods of ancient Chinese, especially in the Pre-Qin Dynasty, and has special grammatical functions which are different from the flexible use for rhetorical effect in contemporary Chinese. It is important to scientifically describe it to know the characteristics of ancient Chinese, to read ancient literature and to understand the traditional culture.