The chlorine isotopic composition is of great significance for indicating the evaporation degree of salt lake brine, judging the potential of salt and potassium formation and environmental evolution. Due to the special provenance structure and salt-forming environment of the Kunteyi Salt Lake (KSL), a highly representative solid potassium-bearing sulfate mineral—polyhalite (Ca?MgK?(SO?)?·2H?O) was formed, which is associated with halite. However, research on the chlorine isotopic of halite under this special depositional background remains is weak, and whether chlorine isotope will be abnormal as a potassium prospecting index needs further study. In addition, investigations into the evolutionary environmental changes of the KSL, particularly its relationship with the great eastern lake, still require more geochemical evidence for support. Therefore, this study focuses on 25 primary halite samples from the ZK3608 core of the KSL, conducting an analysis of the fractionation characteristics and tracing significance of δ3?Cl. The results show that the δ3?Cl values range from -1.08‰ to 0.45‰, with an average of -0.22‰. The δ37Cl decreases from bottom to top, but it is positively correlated with the content of K+ in halite and polyhalite content. It indicates that KSL has experienced a gradual concentration evolution process since the middle Middle Pleistocene, yet no effective potassium accumulation has occurred. In other words, the special hydrogeological background did cause anomalies in chlorine isotopes of the KSL as an indicator for finding potassium, unlike the traditional negative correlation with indicators such as K?. Furthermore, the δ3?Cl values of halite reflect, to a certain extent, the glacial-scale climatic fluctuations of the KSL since Marine Isotope Stage 8 (MIS8). Specifically, they also response to environmental changes such as the freshening event around 170–180 ka, and the complete closure of the lake after the Late Pleistocene. Based on the phenomenon of “salt accumulation but no potassium accumulation” in the KSL since the middle Middle Pleistocene, as indicated by halite δ3?Cl, a preliminary assessment suggests that this phenomenon may be related to the potassium accumulation mechanism of the salt lake and K? supply, which requires further research to clarify.