Abstract: The resluts of exploration and investigation on deep brine in the anticlines of Qaidam Basin showed the deep brine of Paleogene-Neogene anticlines is rich in lithium, boron, bromine and iodine with a good prospecting potential. To discuss the hydrochemical characteristics and the origin of the deep brine of the Yahu structure in the middle of Qaidam Basin, the hydrochemical and sulfur isotopic characteristics of the deep brine were analyzed. The results show: ①The hydrochemical type of deep brines in the Yahu structure are chloride-type. The K⁺, Na⁺ and Mg²⁺ content is low, indicating no prospecting value, while Li⁺, B₂O₃, Br⁻, I⁻, Sr²⁺ content are high, and the majority are higher than comprehensive evaluation index with a good resources potential. ②The δ³⁴s value of deep brine is 44.00 ~ 46.55‰ with an average value of 45.21‰, which is considered that the δ³⁴s value of brine is caused by the bacterial sulfate reduction under good enclosed conditions. ③The characteristics of ion coefficients and others reveal the deep brine in the study area originated from the ancient lake which was sealed up during the sedimentary period of the Pliocene strata, and a series of metamorphosim and transformation such as water-rock interaction and bacterial reduction occurred during the burial process. Due to Himalayaorogeny, the deep magmatic hydrothermal fluid with high Li and B rose along the deep fault and mix with the brine to form the calcium chloride deep brine.