A thick layer of mirabilite was deposited in the lower part of the potassium-bearing strata of the Mahai Salt Lake in the Qaidam Basin. Unlike potassium salts, which usually formed in dry and hot climate, mirabilite is mostly formed under extremely cold conditions. Therefore, the study of the sedimentary characteristics and genesis of thick-layer mirabilite is of great significance for understanding the evolution of ancient environments and their impact on potassium salt mineralization. The article takes the stratigraphic profile of thick-layer mirabilite developed in the Mahai mining area of the Qaidam Basin as the research object, through systematic sampling, thin section identification, chemical composition, mineral type, and sulfur isotope analysis were carried out to study the changes in the chemical composition, mineral type, and sulfur isotope of the profile. The results show that the salt minerals in this section are main stone salt, gypsum, mirabilite, and glauberite, the maximum deposition age of the mirabilite layer is 275.3 ±47.9 ka BP, the research section shows the characteristic of evolution from thick-layer pure mirabilite to gypsum-bearing stone salt from bottom to top. The lower part is mirabilite layer, and glauberite appears in the middle and lower parts. The highest content of stone salt is in the middle and upper parts, while the content of stone salt in the upper part relatively decreases and the content of gypsum increases. Correspondingly, the content of potassium, magnesium, and calcium in the chemical composition of sediments steadily increases, while the overall content of sulfate ions and sodium decreases. During the period of thick-layer mirabilite deposition, Mahai Salt Lake was in a continuously dry and cold climate, which was beneficial to the continuous enrichment and increase of potassium and magnesium in the brine of the salt lake, which laid a foundation for the formation of solid potassium ore through local enrichment.