The traditional study argues that the ancient Chinese “Qi” which used between the subject and the predicate as modal particle, “Qi” can mean many tone such as speculation, judgment, doubt, rhetorical question, order, request, etc., but a modal particle can mean so many different tone is puzzling. This may indicate that the previous study incorrectly attributes the function of the sentence to the function of “Qi”, although the study is meticulous but does not grasp the core function of “Qi” that used between the subject and the subject. For getting the core function of “Qi” between the subject and the subject, we may need to jump out of the limitations of the sentence and stand in the larger language unit - the perspective of the discourse to research. From the textual structure, ancient Chinese sentences with “Qi” mainly appear in the argument of the proof structure, “Qi” can be called “argument markers”. In a specific context, a clause with “Qi” can be an expression of an argument, or it can be an argument for asking others or themselves to perform their own, or to be skeptical of the argument, or to reinforce the argument. The whole sentence will show the judgment, speculation, doubt, imperatives and other tone, but that is the tone of the sentence, rather than “Qi”. The core function of “Qi” is judgement,this function is evolved from the pronoun function. “Qi” and “Shi” have the same evolution, but “Shi” followed by noun components, which prompted “Shi” evolved to copula, but “Qi” followed by verb component, the noun components and verb compents behind of “Qi” can component subject predicate relation, “Qi” in the syntax is difficult to evolve into a typical copula, but “Qi” in fact have judgement function. From the perspective of discourse structure analysis of textual function words is the new method and the study of function words, need special attention.