The forming environment，genesis and distribution rule of the Oligocene lacustrine gypsum-salt rock in western Qaidam Basin have been not clear for a long time，which hinders the progress of petroleum exploration. In order to solve these problems，the sedimentary petrology and geochemical methods were used to study the types and characteristics of gypsum-salt rock，the nature of water bodies during deposition，temporal and spatial distribution and development modes. The results show that the Oligocene gypsum-salt rock in the lower Ganchaigou Formation in western Qaidam Basin is mainly layered halite，followed by gypsum rock，a little glauberite and mirabilite，and it is well euhedral and formed during primary deposition. The gypsum rock is mainly developed at the late stage of water regressive deposition. The sedimentary paleoenvironment has the characteristics of semi-saline and saline brackish and deep water，anoxic water body and low temperature.Its formation mechanism is that the crystalline precipitates due to the evaporation and concentration in a supersaturated lake. The vertical distribution of the gypsum rock is controlled by the fluctuation of lake level. There are 6 key salt-forming periods. The gypsum-salt rock is confined in a zonal distribution in the plane with multiple sub salt-forming centers，which is controlled by the paleomorphology of the lake bottom. Based on the evolution of the Oligocene water body of the lake basin in western Qaidam Basin，there are mainly two development model-the late salinization stage and the wide salt lake stage. In the early stage，the development of gypsum-salt rock mainly formed in the depression area with a limited distribution，while in the late stage，the gypsum-salt rock was distributed widely，extending to the low amplitude uplift area.