Exploring the spatial distribution pattern of mountain tree-shrub patches and the relationship between their diversity and altitude are important for revealing the environmental adaptability of regional plants and implementing regional biodiversity conservation and ecosystem restoration. Based on the data of the third forestry census, this paper analyzed the spatial distribution pattern of tree and shrub patches and their diversity variation along the altitude gradient in the transition zone between the first and second steps of the northern Hengduan Mountains using the method of geographic information system. The results showed that:① There were 103 tree-shrub species belong 37 families and 72 genera in the transition zone. The number of tree species (56 species) was slightly more than that of shrub (47 species). The proportion of native species was 99.03% and the proportion of endemic species in China was 22.33%. The dominant tree species here were Abies fabri, Picea asperata, Pinus densata, Quercus semecarpifolia, Betula potaninii, and Larix gmelinii, while the dominant shrubs in the transition zone were Rhododendron lapponicum, Quercus aquifolioides, Salix cupularis, Rhododendron simsii, Lonicera japonica, and Rosa multiflora. ② The horizontal pattern showed that the total area of shrub patch was about 1.24 times that of tree patch, showing obvious dominance. However, the fragmentation degree of shrub patches was higher than that of trees. Shrub patches were distributed in the northern, central, and western transition zone. The tree patches distributed in bands along the Jinsha River, Yalong River and Dadu River. The area of shrub patches was higher than that of tree patches in the low-, high-, and extremely high-altitude areas of the transition zone, but the area of tree patches was about 1.80 times that of shrub patches in the middle-altitude area. ③ Diversity analysis showed that the diversity of tree-shrub in the transition zone was good, and the tree diversity was better than that of shrub. The eastern region of the transition zone had the best diversity of trees and shrubs. Tree diversity in Luding and Jiulong counties were optimal, and the shrub diversity in Luding and Danba counties was optimal. With the increasing altitude gradient, PR(Patch Richness) values of tree-shrub showed a unimodal trend, and SHDI (Shannon's Diversity Index) and SHEI(Shannon's Evenness Index) values of tree-shrub showed a bimodal pattern, but the peaks of these three indices occurred in the middle-altitude area. The diversity variation trend was consistent with the type of "intermediate height expansion". Both SHEI value of shrub and the total SHEI value of tree-shrub showed a significantly negative linear correlation with the increasing altitude.