The potassium-magnesium salt deposit in Khammouan Province of Laos is located in eastern Khorat Basin which belongs to upper Cretaceous salt sedimentary basin. The formation of the deposit underwent three sedimentary cycles which formed three salt sections, i.e., upper, middle and lower sections, and every salt section includes two layers, namely salt layer and clay layer. The ore is located in the lower salt layer of lower salt section. This paper mainly studied the mineralogical and geochemical characteristics of the lower salt layer based on ZK318 well. The results show that halite, sylvite and carnallite are main minerals, with small amounts of boracite and anhydrite. In combination with the characteristics of minerals, major elements and trace elements, the lower salt layer can be divided into five layers in upward order of halite layer→ sylvite layer→ carnallite layer→ thin halite layer→ upper sylvite layer. Bromine (Br) values (higher than 200×10^-6) show that the brine came from sea water, and carnallite occurred at the highest stage of brine concentration, as shown by its highest Br value of 2 572×10^-6 among five layers. Boron content showed a gradual increase from halite layer through sylvite layer to carnallite layer, suggesting that boron content is in linear correlation with salinity of the brine. Boron value of the upper sylvite layer is up to 890×10^-6, which is consistent with the existence of boracite mineral revealed by XRD, indicating that ore brine came from sea water and deep hydrothermal solution. In addition, according to the evaporative order of halide brine, the order of crystallization was halite→sylvite→carnallite, and carnallite seemed to be the last sedimentary period in halide brine. However, in ZK318 well, there appears a thin layer of halite above the carnallite layer, which shows that it underwent a short period of fresh water supply which result in the decrease of brine concentration in the basin, but soon the concentration continued to increase with the replenishment of deep hydrothermal solution, followed by the precipitation of the upper sylvite layer. Thus, the sedimentation of the lower salt layer was over. Therefore, the upper and lower sylvite layers are both primary sedimentary layers because they overlie the halite layer directly, and have the same content of boron, being 1 790×10^-6 and 1 792×10^-6, respectively. In summary, potassium sources of the ore deposit were sea water and deep hydrothermal solution, and two sylvite layers in the well are both primary sediments.