This paper compared and analyzed the phonological system reflected in Mengguziyun and Hebingzixuejiyun centering on yinshengyun. Mengguziyun is an official rhyme book compiled by the Yuan court to provide an orthography for dushuyin of Northen Dialect in Yuan dynasty and Hebingzixuejiyun is a rhyme book compiled in Ming Dynasty to provide an orthography for dushuyin of Northen Dialect at that time. Assimilation is a sound change that leads the changes in the Chinese phonemes system. Medial(yuntou) was influenced by features of initial(shengmu) and become more similar to them, like shejianhua. And nucleus(yunfu) was influenced by features of medial and becomes more higher, like open [a] were raised to open-mid [ε].