In this paper, the author induces exclusive rules and harmony rules between medial onglides and the other phonological elements as revealed in Modern Pekinese, Middle Chinese and the phonological changes from Middle Chinese to Modern Pekinese. Moreover, the paper surmises the phonetical reasons and the historical causes of these relationships. As a matter of fact, only initial consonants and ending consonants/vowels have close relationships with medial onglides, while principal vowels and tones rarely have special relationships with them.