It was believed that there were not prospective oil source rocks (with high organic carbon contents and better hydrocarbon potentials) deposited in the evaporite lacustrine of the Qaidam basin in Tertiary. Based on the investigations of co-sedimentation of the evaporites and source rocks in age of Oligocene and Miocene in the western Qaidam basin, the authors represented that the prospective oil source rocks were just distributed stratigraphically and tectonically in the locations where halite and gypsum were deposited. A genetic model was postulated for evaporites and source rocks, which was described as “big shallow-water saline lake surrounding small deep-water brine lake”. The authors believed that water depth controlled both on deposition of the evaporites and the accumulation of organic matter in the salt-lake environment. And a theoretical model for forming of oil source rocks in the salt environment was presented in this paper, which was a useful tool for finding prospective source rocks in the Qaidam basin.