This paper is the result of regional geological survey. [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.