Abstract: Based on a field survey conducted during the Third Xinjiang Scientific Expedition, this study compiled a comprehensive list of seed plants in the valley forests of the Irtysh River basin in Xinjiang and analyzed their floristic characteristics using floristic geography methods. Results identified 403 seed plants belonging to 55 families and 228 genera, representing 48.67%, 29.73%, and 10.84% of seed plant families, genera, and species in Xinjiang, respectively, highlighting rich plant diversity. Herbaceous plants dominated the flora, accounting for 80.40% of the total species, whereas tree species were relatively limited, with Salicaceae serving as the dominant constructive and characteristic family within the communities. Furthermore, shrubs were notably absent, representing an important aspect of the valley forests. Geographically, the families and genera were classified into seven and 25 distribution types, respectively, with 59.09% of non-cosmopolitan families and 92.90% of non-cosmopolitan genera exhibiting temperate distribution patterns. The interaction between plant species and environmental factors has resulted in transitional adaptations, including xerophytes and hyper-xerophytes suited to arid conditions and cold-adapted hygrophytes thriving in mountain forests and alpine meadows. These valley forests act as a crucial regional species pool with significant conservation value. Moreover, the study also revealed that valley forests can be classified as piedmont or mountain according to community composition, dominant tree species, topography, and geomorphology.