Objectives: To discuss the hydrochemical characteristics and the genesis of gravel-type deep brine of the Mahai area in the northern margin of Qaidam Basin. Methods: The hydrochemical and H-O, S, Li isotopic characteristics of the Pleistocene deep brine were analyzed. Results: The results show: ①The hydrochemical type of brines are chloride-type. The K+ and Na+ of the content element are high, and the majority are higher than comprehensive evaluation index with a good resources potential; ② The ion characteristic coefficients and isotopic composition of brine indicate the brine has the characteristics of salt rock dissolution, clay mineral adsorption, strong evaporative concentration, and relatively closed conditions . Conclusions: The brine is the product of a series of geological processes such as water-rock reactions, leaching, adsorption, and evaporation concentration by atmospheric precipitation. In the transition zone between the terminal salt lake and the alluvial fan, with the increasing pressure of overlying formation, the residual ancient water was moved to the sand-gravel layers with low pressure in front of the mountains, and eventually infiltrates to the deep sand-gravel layers, then formed the gravel-type deep pore brine deposit under the condition of relative sealing.