Abstract: The linear regression models and variation partitioning method were used to analyze the relative and pure effects of area, climate, and boundary constraint on altitudinal patterns of plant species diversity in the Baishuijiang Natural Reserve. The results showed that diversities of overall plant species and different group species exhibited humpedshaped patterns and the peaks were below the midpoint of the elevation span. The patterns were shaped by all factors together. The explanatory power of waterenergy  hypothesis for the pattern was the strongest. Area acted as a secondary factor the patterns. Boundary constraint effect was a complementary mechanism to explain the plant species diversity altitudinal patterns in Baishuijiang region. The explanatory powers of boundary constraint for differentrange species were clearly different. With the increase of species range, boundary constraint showed an increasingly strong correlation with richness patterns.