Abies georgei var.smithii, which grows at elevations between 3600m and 4400m, is the main constructive tree species of the upper timberline in southeast of Tibet as well as the dominant species of the upper timberline on the shaded slopes. This paper presents the results of our investigation of population structure and dynamics, and timberline features of Abies georgei var.smithii populations in the transition zone of the timberline community of Mt. Sejila. The upper elevation limit of the closed forests was found to be 4320m. Two types of timberline were exhibited. On the sunny side of the mountain, the timberline exhibited a gradual change type whereas on the shaded side, there was a sharp change in timberline. The main species within these two types of timberlines were Sabina saltuaria and Abies georgei var.smithii, respectively. The timberline elevations on the sunny side and shaded side of the mountain were 4570m and 4390m, respectively. The respective widths of the timberlines on the sunny slope and the shaded sides were 250m and 70m. The optimal range of distribution of Abies georgei var.smithii populations on the shaded side was 3700-3800m. The distribution of diameter classes of the Abies georgei var.smithii population appeared as reverse “J shape” and its density was about 380/hm2. The age structure of the population was pyramid-shape, suggesting this was an expanding population. The static life table and the survival curve showed that the populations had experienced severe environment changes and self-thinning in year 20 and years 60-100 following their establishment. They also demonstrated death fluctuation related to the environmental changes at a later stage. The physiological longevity of Abies georgei var.smithii was about 200 years, and its ultimate longevity was around 400 years.