The National Basic Resources Investigation Program "Integrated Scientific Investigation of the North-South Transitional Zone" introduced the concept of north-south transitional zone (Qinling-Daba Mountains). Variation of north-south series in vegetation of the Qinling-Daba Mountains is of great significance to reveal the transition, diversity and complexity of the regional structure of China's north-south transitional zone. Based on the survey data of plant communities, in this study, the eastern Qinling-Daba Mountains is divided into four geography units: north flank of eastern Qinling Mountains (EQMN), south flank of eastern Qinling Mountains (EQMS), north flank of eastern Daba Mountains (EBMN), south flank of eastern Daba Mountains (EBMS). The regional climate divisions on levels of plant species, plant community structure and species richness are explored. The results show that: (1) On plant species level, there are mainly north plants in EQMN, evergreen species and fewer north plants in EQMS. Then to the eastern Daba Mountains, there are mainly south plants which like wetness and heat. (2) On plant community structure level, there are 4 formations (3 northern formations, 0 southern formation, 1 widespread formation) in EQMN, 6 formations (3 northern formations, 1 southern formation, 2 widespread formations) in EQMS, 4 formations (0 northern formation, 2 southern formations, 2 widespread formations) in EBMN, 3 formations (0 northern formation, 3 southern formations, 0 widespread formation) in EBMS. In terms of the numbers and properties of formation, there is a mixture of northern and southern formations only in EQMS. On species richness level, the diversity of family, genus and species decreases with the increase of latitude, but the mixture of north and south plants occurred in the south flank of eastern Qinling Mountains. This paper studies the variation of series in vegetation, which increases the scientificity of determining north-south dividing line, and shows that the south flank of eastern Qinling Mountains is more suitable as the warm temperate-subtropical zones dividing line.