Abstract: Potassium and magnesium salt deposit mainly enriches in the lower gypsum rock salt segment of Tagong Formation in Vientiane basin, Laos. The deposit is mainly composed of salt rock, carnallite, potassium salt, carnallite bearing potassium and a small amount of spar etc, and it belongs to single-layer structure. Trace elements of potassium and magnesium salt deposit such as Br and Rb are directly related to chloride, and therefore work as sign of potassium deposit prospecting in the Vientiane basin. These elements can reflected the development process of the original concentration of salt solution in certain degree. Sr and B with stable geochemical nature concentrate in the brine solution with the increase of brine concentration, so they may be indicative for potassium layer. It is confirm that the Vientiane basin brine comes from the southern Khorat basin by the analysis of trace elements, and it's possible that there is also surface water from deep fracture brine supply. This conclusion can be quite helpful to enrich the ancient solid potassium salt mineralization theory and to guide the research of Lanping-Simao basin evaporite salt and other potash deposits prospecting.