Abstract: The north-south transitional zone of China is physiogeographically characterized by the development of evergreen and deciduous broad-leaved mixed forests. However, there have been many differences in understanding the internal structure and geographical zonality of this type of mixed forest. Based on field vegetation survey data obtained from the national project of "Integrated Scientific Investigation of the North-South Transitional Zone in China", the changes in woody plant composition westly along the southern piedmont plains of the Qinling Mountains, and also along three north-south crossing transects (Sanmenxia-Shennongjia; Xi'an-Dazhou; Tianshui-Guangyuan) are analyzed. The results show that, in the east-west direction, the importance value (Ⅳ) of evergreen broad-leaved woody species is relatively high (27.57%) only in the Xunyang section, and it is very low or even nearly zero in other sections, especially in the Diebu-Bailongjiang section at the westernmost end and the Hanzhong-Yangxian section where human activities are concentrated. The plant species number, abundance and the Ⅳ of evergreen broad-leaved woody plants all increase gradually from north to south, but the Ⅳ falls below 20%. On the whole, the deciduous broad-leaved woody species occupy absolute advantage in the Qinling-Daba mountains; while the evergreen broad-leaved species appear apparently only in the south of the Daba mountains. This shows that vegetation in the Qinling-Daba mountains exhibits a strong transitional nature, mainly inclining towards the warm temperate deciduous broad-leaved component. To the east of the Qinling-Daba mountains, the vast areas on both banks of the Huaihe river are more affected by low temperature in winter, which leads to the formation of a broad transitional zone with complex internal structure and strong transition in the middle of Chinese mainland, which is obviously different both from the warm temperate deciduous broad-leaved zone and from the subtropical evergreen broad-leaved forest zone, and thus should be treated as the highest-level physiogeographical zone.