It has been more than 40 years since Yunmeng Qin bamboo slips were unearthed. As the first batch of Qin bamboo slips unearthed, it is close to the truth of the Qing dynasty. Because the unearthed bamboo slips are clear and easily to recognize, most of them are complete, which provides favorable conditions for the in-depth study of Yunmeng Qin bamboo slips. To start with, the aim of this paper is to explore the special words used in the development period of modal particles, to clarify the development context of the evolution of modal particles, and to sort out the internal differences of the evolution of modal particles in Yunmeng Qin bamboo slips. We give a possibility for Yi (殹) and Ye (也), which seems to alternate between Yunmeng Qin bamboo slips. Secondly, we found the factors to these change: dialect, country, written forms and customs. Also, we set many examples to prove Yi (殹) and Ye (也) in Yunmeng Qin bamboo slips truly alternative. The time of Yunmeng Qin bamboo slips was written at about the end of the Warring States period to the early Qin Dynasty, which was the time when the words were not solidified. In this period, the use of words was chaotic, and modal particles showed different use of words because of their distinctive regional characteristics, that is, the diversification of writing forms. Especially, Yi (殹) and Ye (也) were two modal particles in Yunmeng Qin bamboo slips. As a Qin character, commonly seen in Yunmeng Qin bamboo slips, Yi (殹) is often used as a modal particle in the slips of Qing dynasty, which has been discussed by senior scholars such as Li Xueqin and Chen Zhaorong. Mr. Li talked about Yi (殹) and “habitually seen in Qin Bamboo Slips” in his article “an Investigation of Ancient Philology of Qin Bamboo Slips”; When discussing the evolution of Qin characters, Mr Chen interpreted Yi (殹) as a mood auxiliary word with both regional and contemporary characteristics.