The Tarim basin was the eastern extension of the Paratethys Sea during the Paleogene, with the Kuqa sub- basin located in the northeastern part of this basin. It experienced a sedimentary evolution from shallow marine, marine terrigenous to continent, resulting in deposition of giant evaporites. It has become an important base for oil- gas production and a key exploration area for potash deposits, thus having attracted wide attention from a large number of geologists. However, there are still some controversies and uncertainties about the source and degree of evaporation and concentration of Eocene ancient brine in the Kuqa Basin. Therefore, this study, based on observation of petrology and mineralogy, analysis of XRD and SEM- EDS and stable isotope characteristics, analyzed depositional environment, supply origin and degree of evaporation and concentration of the evaporite. Petrography analyses of evaporite show that it deposited in a salt lake environment mixed with addition of proximal mud. The anhydrite δ34S values are characteristic of certain marine source supplement. The δD and δ18O values of primary fluid inclusions from halite range from -118.1‰ to -95.3‰ and from 2.5‰ to 5.7‰, respectively, and the evaporation trend line indicates that the ancient brine had a high degree of evaporation and concentration. Although experiencing three transient freshwater influxes, the evolution of the ancient salt lakes did not affect the precipitation of potassium and magnesium saline minerals with continuous evaporation of ancient salt lakes, and the corresponding potassium saline minerals were precipitated when the δ18 O values were positive. It can be speculated that Eocene salt- bearing series strata in the Kuqa basin has favorable potential for potash formation.