Abstract: In ancient Chinese, the usage of lai is extremely complex. Only under its auxiliary word attribute, it can be subdivided into six or seven different usages. However, most of these usages have not been used in the common modern Chinese, including lai used as tense auxiliary. The survey found that after the emergence of the tense auxiliary word lai, its use did not show a rising trend in the following periods:Tang and Five Dynasties, Song and Yuan Dynasties, Ming and Qing Dynasties. However, it gradually declined in the standard Chinese during the diachronic development. Today, it is only used in some southern dialects. As a tense auxiliary word, there are many reasons for the decline of lai, including not only internal reasons such as the heavy burden of its own function and too many restrictions in its use, but also external reasons such as the competition among members of the system.