The potassium-magnesium salt deposit of Khammouan Province in Laos is located in the eastern part of Khorat Basin. The main salt minerals of the deposit are halite, sylvite, carnallite and anhydrite, and the boron minerals in the deposit are of small or even trace amounts. The identification of trace boron minerals in the lower salt layer of ZK318 borehole in the deposit was studied in order to complete the mineral assemblage type. In order to identify the type of boron minerals accurately, the authors conducted XRD analysis of insoluble substances collected through the soluble experiment of bulk rock samples. The result shows that the boron minerals in insoluble substances are mainly boracite （55 % ）, and subordinately anorthic hilgardite-3A （15%）. Hilgardite-3A belongs to trace mineral because its content is less than the detection limit of XRD （1% ） so that there is no diffractive peaks in the XRD pattern of bulk rock samples.