Judging the evaporation concentration stage of salt lake brine is of great significance for indicating potassium formation. The low Br/Cl ratio in the Eocene salt- bearing strata in the Kuqa basin reveals the limitation of this traditional method of finding potassium geochemical indicators. However, thanks to the rapid progress in applications of stable isotopes in recent years, chlorine isotopic composition can be a good indicator of evaporation stage of salt- bearing. This paper takes 13 primary rock salt from core DZK01 of Kuqa basin as the research object. The analysis results show that δ37Cl values are all negative values, ranging from -1. 20‰ to -0. 51‰ and KCl content is between 0. 13% and 0. 29%. From bottom to top in the section, the values of δ37Cl tend to increase, and KCl content tends to decrease indicating that the middle Eocene ancient brine in the Kuqa salt basin has undergone a gradual dilution evolution process.The early stage brine had a higher degree of evaporative concentration, and sylvite or potassium- containing minerals were precipitated. The late stage brine had a lower degree of evaporative concentration, and few or no sylvite or potassium- containing minerals arefound. In addition, salt- forming materials of middle Eocene evaporite series in the Kuqa Salt Basin are mainly marine sources, with higher degree of evaporation and concentration indicating a superior prospect for potassium formation.