This article mainly explores the grammaticalization and subjectivization of the modality adverb "fanzheng(反正)" in modern Chinese, and analyzes the reasons for its continuous evolution. First, from the diachronic perspective, the modal adverb "fanzheng" is not the product of word-formation mechanisms. It first appeared in the Song Dynasty as a coordinate phrase which expressed 'negative (fan)' and 'positive (zheng)'. Later, under the mechanism of metaphor, "fanzheng" was used very frequently to express abstract meaning. The boundary between "fan" and "zheng" disappeared, moreover, "fanzheng" became a dvandva compound in this transmutation. During the process of lexicalization, "fanzheng" was influenced by "zaowan(早晩)" and "sihuo(死活)", which were composed of two antonymous adjectival morphemes and grammaticalized to modal adverb, and it finally grammaticized by analogy mechanism in the Qing Dynasty. From the synchronic perspective, the usage of modal adverb "fanzheng" in modern Chinese can be divided into two types. One is as a clause-internal adverb, which has narrow scope; the other is used as a sentential adverb, with the scope gradually extending to the whole sentence. In the second usage, we found that "fanzheng" is phonologically independent and its textual function is enhanced, which has the function of connecting a previous sentence. With these features, "fanzheng" becomes a discourse marker at the conversation level, for showing speaker's attitude, expressing the speaker's subjective conclusions. The speakers use the discourse marker "fanzheng" to facilitate the listener to understand their intentions immediately and interact with them. Furthermore, the discourse marker "fanzheng" usually appears in uncertain contexts. In these contexts, "fanzheng" appears frequently and is used as a filler, reflecting that the speaker is thinking and has not complete the speech. From modal adverb to discourse marker to filler, each stage of "fanzheng" is a case of grammaticalization and subjectification (including intersubjectification).