This paper is the result of environmental geological survey engineering.[Objective] The study of paleo-environmental changes of Mid-to Late-Pleistocene in northwest Tarim Basin is relatively weak. As a sensitive area to climate and environmental change in Asia, the paleoenvironment study of the Southwest Tianshan is of great significance for predicting future climate. The succession law of palaeo-vegetation indicated by the characteristics of palynological assemblage has become an important problem to be solved urgently in paleoclimate research in this area. [Methods] Based on palynological identification and photoluminescence dating of typical river terrace samples from Sepayi-Keping area, Xinjiang, the characteristics of palynological assemblages in the Mid-to Late-Pleistocene and their implications for paleoclimate were discussed in this paper. [Results] Three palynological assemblages of Mid-to Late- Pleistocene were divided. Palynological assemblage zone Ⅰ: Quercus-Graminae- Chenopodiaceae; Ⅱ: Ephedra-Betula-Labiatae; Ⅲ: Pinus-Ephedra-Chenopodiaceae. [Conclusions] They indicate the arid climate of the Southwest Tianshan and the fluctuation trend (dry and cold-warm and wet-dry and cold) during Mid-to Late-Pleistocene. During 65-16 ka before present, there was an obvious humid climate stage with abundant precipitation. The terraces formed by river downcutting were controlled by both tectonic and climatic factors.