Abstract: We clarified forest constructive species in transition area, and put ‘Intermediate expansion hypothesis’ to the test. We took the highest peak Juxianfeng in Tianjin of China as study site, and set transect sampling along elevation vertically. The results were as follows:(1) Change rules of dominant species in every layer differed from each other. In tree layer, dominant species in lower area were Quercus aliena and Q.variabilis. Increasing with altitude, Juglans mandshurica gradually became dominant, mixed with Pinus tabuliformis.Q.mongolica played an important role in community. Until top of the mountain, the plant communities were Tilia amurensis-Q.mongolica mixed forests. However, Q.aliena and Q.variabilis became companion species at the same time. Quercus distributed everywhere and were keystone species in this transition area. Deutzia parviflora was absolute dominant species in shrub layer within all altitude. Dominant species in herb layer showed disordered. (2) α diversity indexes changed vertically. Total species richness S reached the maximum in 600-700 m. The peak of Simpson index appeared in 800-900 m which was in middle high altitude, suggesting more information and complexity. Shannon-Wiener and Pielou indexes showed the same law that was bimodal pattern, namely numerical value in 700-800 m was lower than both two sides. Therefore, our results didn't support ‘Intermediate expansion hypothesis’ in transition area. Our study enriched the ecology theory in mountain transition region, and provided plant diversity information of the north mountain in China.