This paper focuses on the topic marker "bu zai hua xia" in old Mandarin Vernacular literature discourse, whose basic semantic meaning is "to stop talking about a topic" and pragmatic function is to mark the end of a topic, expressing the speaker's identity and subjective attitude toward the topic. It expresses the speaker's identity and subjective attitude toward the topic as "insignificant and not worth mentioning", reflecting the speaker's care for the listener's comprehension activity. The article also examines the development and evolution of "bu zai hua xia", arguing that since the Ming and Qing dynasties, "bu zai hua xia" has been subjectivized in the language of the subject, with the semantics derived as "things are trivial and not worth mentioning" and "not a problem". In Modern Chinese, the topic marker "bu zai hua xia" has died out, while the idiom "bu zai hua xia" is still active in various discourses.