The varved lake sediments are one of important archives for high-resolution paleoclimate and paleoenvironment studies, and studying their sedimentary mechanism could provide an important basis for high-resolution paleoclimatic and paleoenvironment reconstructions. In this paper, observation of petrographic thin sections under microscope, and micro-area analyses, such as scanning electron microscope, as well as X-ray diffraction analysis, and synchrotron radiation X-ray fluorescence analysis, are used to study morphology, deposition and mineral characteristics of varved Lake Kusai sediments in the Hoh Xil region of Northern Qinghai-Tibetan Plateau, and thus to investigate their sedimentary process and mechanism. The results show that Varved Kusai Lake sediments are very steadily and well rhythmically laminated with interaction of light and dark layer.. The dark layers are made of grey brown or black brown coarse material with middle rounded and poor sorted grains, composed of quartz, feldspar, mica, and clay minerals, among which quartz accounts for about 73%. The features of the dark layers are quite similar to those of aeolian deposits. The light layers are made of gray and white thin material, composed of quartz, feldspar, muscovite, and clay minerals. The light layers contain authigene minerals such as aragonite. Meanwhile the light layers include many half-moon-shaped ostracod shells and cladocera remains. Thus, we suggest that dark layers are formed by coarse sand and silt that were deposited on lake ice by aeolian processes during winter when the temperature is low and the wind is strong. Upon ice thawing during summer, these sands and silts may be released and deposited into lake sediments to form dark layers (coarse particle layer). We interpret the light layers (fine particle layer) to represent summer season accumulations, when thin materials are transported to the lake center by high discharge caused by snow melting and high precipitation. Authigene minerals such as aragonite deposited under the condition of high evaporation, and ostracodes and cladogram grew in warm temperature conditions. In Lake Kusai, authigene biological action and chemical action enhance, biological activity strengthen. Thus the light layers include thin materials, biological debris and authigenic carbonate precipitation.