“是必”作为一个语气副词,在近代汉语时期一度比较活跃,但进入现代汉语时期,由于自身的弱势和同类词的竞争等原因,已经消失。在当代汉语时期,又再次出现在人们的视野中。本文主要从共时和历时两个角度考察、分析“是必”产生、发展、消亡以及再度出现的过程和原因,并指出“是必”的退隐和再现是由语言系统的自我调节所决定的。 As a modal adverb, it was once active in modern Chinese period, but it has vanished due to its own weakness and competition of similar words. In the period of contemporary Chinese, it appears again in people’s vision. This paper mainly studies from the synchronic and diachronic perspectives, analyzes the process and cause of the emergence, development, demise and reappearance of “must ”, and points out that the retirement and reproduction of "must Regulated by the decision.