Lake Qinghai is situated on the northeastern Tibetan Plateau, which has played an important role in research fields about climate environment evolution and recent uplift of the Tibetan Plateau, as well as questions about regional tectonics and the evolution of sedimentary basins in this area.Yilangjian core, drilled from the Yilangjian terrace on the southern shore in 2005, is located in the southern depression of Lake Qinghai.A 626.39 m long core from Yilangjian area reveals sedimentary stratum stable and continuous generally, and it can be easily divided into two parts according to lithology characteristcs of the core.The upper 584.64 m of core consists of continuous lacustrine silty clay interrupted party by coarse sand and silt, and the underneath core from 586.64 m to 626.39 m in depth is the yellow-brown earth sediments, which is different from the upper lacustrine sediments obviously.In order to make clear the origin of the yellow-brown earth sediment on the bottom of Yilangjian core, firstly, the yellow-brown earth sediment lithology characteristic were observed, and most earth sediment are underlain by carbonate concretion horizons similar to those in Quaternary loess.Secondly, we present results of grain-size analyses of the yellow-brown earth sediment on the bottom of Yilangjian core from Lake Qinghai, and in particular their grain-size distribution is compared with that of typical aeolian loess—paleosol, red clay, as well as lacustrine and fluvial sediments.It appears from the sedimentological evidence that the major part of the yellow-brown earth sediment is of aeolian origin.It is rather similar in some of its properties to the red clay.Thirdly, Quartz grain morphology in earth sediment shows clearly subangular to subrounded shape and collision pits which are characteristic of aeolian dust deposits, and are clearly different from Quartz grain morphology in typical lacustrine sediments in upper core.The study into the stratigraphy of YL drilling core indicates that the typical lacustrine deposit starts at the depth of 586.64 m, and the paleomagnetic age at this depth is 4.64 Ma.Therefore, the abrupt climate transformation event had been defined from the change from aeolian dust deposits to lacustrine sediments, which occurred at about 4.64 Ma.Meanwhile, it is inferred that, in modern meanings, Lake Qinghai may also come into being at 4.64 Ma.