The Mailong gold deposit is located in the eastern section of the East Kunlun orogenic belt and is one of the recently discovered important gold polymetallic deposits in the Qinhai Gouli region. The primary host rocks of the Mailong gold deposit consist of intermediate-acid intrusive rocks from the Varisian and Indosinian periods, as well as the Precambrian Jinshuikou Group, with mineralization controlled by northeast and northwest faults. The alteration of the host rocks is mainly characterized by silicification, sericitization, chloritization, and carbonatization. Based on the cross-cutting relationships of the veins, the hydrothermal mineralization of the gold deposit can be categorized into three stages: the quartz-pyrite stage, the quartzpolymetallic sulfide stage, and the quartz-carbonate stage. Microthermometry of fluid inclusions indicates that the Mailong gold deposit belongs to a low-density (0.73–0.86 g/cm3 ), medium-temperature (240–340 ℃), and medium-salinity (4.01–10.74 wt% NaCl) NaCl-CO2-H2O fluid system. The C-H-O isotopic analysis suggests that the mineralizing fluids is derived from magmatic water, with later contributions from atmospheric precipitation. In-situ S isotopic results indicate that the mineralizing materials mainly derive from igneous rocks. A comprehensive analysis concludes that the Mailong gold deposit is a mesothermal hydrothermal vein-type gold deposit controlled by structural factors.