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The Ili River Valley is located in western Xinjiang as a ’wet island’ in an arid region, and is rich with unique plant species. In this paper, TWINSPAN (two-way indicator species analysis) and DCCA (detrended canonical correspondence analysis) were performed to analyze the vegetation patterns and influential factors based on data from a field investigation in the mountainous region of the valley. 259 species were recorded in the 85 plots, up to 235 species all appeared in the herb layer, and species of woody plants were very limited. The species composition varied widely between the northern slope and the southern slope. 35 plots on the northern slope were clustered into 10 groups, and 50 plots on the southern slope were clustered into 11 groupes. The DCCA ordination indicated that elevation as a compound factor was the critical driver of species composition on both the slopes. The vegetation patterns were also related to slope gradient, aspect, soil water, soil pH, organic matter and total nitrogen. Available phosphorus and available potassium were important factors on the southern slope only, maybe because of the appearance of shrubs there. The proportion of variation explained was 49.54% on the northern slope and 41.08% on the southern slope. Edaphic variables were more important than topographical variables on the northern slope, while topographical variables seemed to be more influential on the southern slope.