The areas sharing the same sound change are usually in accordance with historical administrational regions as a result of the impact of the dominant culture. In this paper, with examples such as the modern reflexes of division 2, division 3 and 4 of Middle Chinese Jianmu Kaikou initials, and modern reflexes of Middle Chinese Bingmu initials in level-tone syllables as shown in GIS, the author identifies two critical periods in forming Chinese southeastern dialects: Weijin as the formation period of colloquial readings of Chinese dialects, and Tang, Wudai, Song, and Yuan as the formation period of literary readings of Chinese dialects.