During formation of the Tarim Basin, there repetitively happened seawater invasion for it was ever connected with wide Paleo-mediterranean Sea. At the same time, for the applica-tion of tectonic and paleoenvironmental condition, the basin deposits many sequences of thick strata of evaporites, especially in its two largest subbasins of Kuqa in the north and Shaqa in the southwest. Therefore, the western Tarim Basin has been considered to be a favorable location to prospect ancient sylvinite deposit. But it is unfortunate that the Br×103/Cl ratio, as a common geochemical index to uncover clues of sylvinite deposit, is almost invalid to indicate the stage of ancient brine condensation for very low content of bromine in salt rock samples (less than 10?6 commonly) in the western Tarim Basin evaporites. Fortunately, we noticed recently that the val-ues of chlorine isotope (δ 37Cl value) for salt rock samples follow some clear laws from place to place, which in the Shaqa subbasin, all δ 37Cl values of salt rock samples behaved negatively and on the contrary, they all surprisingly showed positive values in the Kuqa subbasin. Based on conclusions of previous research and referring to the ratios of Br×103/Cl for salt rock or brine samples, we proved that the chlorine isotope (δ 37Cl) could be used as an effective index to find ancient sylvinite deposit in those evaporites rock basin with low content of Br element back-ground, such as the Tarim Basin. Thus according to the new index of chlorine isotope, we con-clude that the concentration of ancient brines formed ancient salt rocks is much more concen-trated in the Shaqa subbasin than that in the Kuqa subbasin. Particularly, the δ 37Cl value of salt rock in Kashi depression of the western Shaqa subbasin is greatly negative (<?1.00‰), which may predicate that the ancient brine has been concentrated to the terminal stage of salt deposit or even the later stage of sylvinite deposit. Then it is possible to find sylvinite-bearing deposits around the western region (periphery of Kashi depression) of the Shaqa subbasin if we carry out more careful exploration in later works. During formation of the Tarim Basin, there was repetitively happened seawater invasion for it was ever connected with wide Paleo-mediterranean Sea. At the same time, for the application-tectonic and paleoenvironmental conditions, the basin deposits many sequences of thick strata of evaporites , especially in its two largest subbasins of Kuqa in the north and Shaqa in the southwest. Therefore, the western Tarim Basin has been considered to be a favorable location to prospect ancient sylvinite deposit. But it is unfortunate that the Br × 103 / Cl ratio , as a common geochemical index to uncover clues of sylvinite deposit, is almost invalid to indicate the stage of ancient brine condensation for very low content of bromine in salt rock samples (less than 10? 6 commonly) in the western Tarim Basin evaporites. Fortunately , we noticed recently that the val-ues of chlorine isotope (δ 37Cl value) for salt rock samples follow some clear laws from place to place, which in the Shaqa subbasin, all δ 37 Cl values ​​of salt rock samples behaved negatively and on the contrary, they all surprisingly showed positive values ​​in the Kuqa subbasin. Based on conclusions of previous research and referring to the ratios of Br × 103 / Cl for salt rock or brine samples, we proved surprisingly that the chlorine isotope (δ 37Cl) could be used as an effective index to find ancient sylvinite deposit in those evaporites rock basin with low content of Br element back-ground, such as the Tarim Basin. According to the new index of chlorine isotope , we con-clude that the concentration of ancient brines formed ancient salt rocks is much more concen-trated in the Shaqa subbasin than that in the Kuqa subbasin. Particularly, the δ 37Cl value of salt rock in Kashi depression of the western Shaqa subbasin is greatly negative (<? 1.00 ‰), which may predicate that the ancient brine has been concentrated to the terminal stage of salt deposit or even the later stage of sylvinite deposit. Then it is possible to find sylvinite-b earing deposits around the western region (periphery of Kashi depression) of the Shaqa subbasin if we carry out more careful exploration in later works.