Abstract: The hydrothermal period related to molybdenum mineralization of the Yuchiling Mo deposit can be divided into 4 stages, i.e., quartz-potash feldspar stage, molybdenite-quartz stage, sericite-quartz alteration stage and quartz-carbonation stage. Fluid inclusion studies and isotope geochemical analysis of the 4 stages show that the ore-forming fluids of the 4 stages changed physicochemically from magmatic fluid characterized by high temperature, salinity, oxygen fugacity, CO2 and high content of metal elements of the earlier stages to meteoric fluid characterized by low temperature, salinity, oxygen fugacity, CO2 and low content of metal elements of the later stages. The ore-forming depth of quartz-potash feldspar stage was about 5.85km, that of the molybdenite-quartz stage was 4.11km, and that of the last stage was near the surface. δ18OH2O shows that the ore-forming fluid was dominated by magmatic water at the early stage, whereas at the late stage, it could be produced by mixed magmatic and meteoric water. The compositions of sulfur isotope show that sulfur might have been derived from the mantle. The oxygen fugacity, pH values and Eh values decreased from early stage to the late stage, and the Eh values suggest a weak reducing environment during the precipitation of the Mo element. Before the main ore-forming stages, Mo element probably existed as H2MoO4 and ion pair, with a small amount of Mo-O-Cl complex. Mo-O-Cl complex was probably predominant when it entered the main ore-forming stages. The boiling and mixing action of the fluid as well as the water-rock interaction and evolution of the fluid from the depth to the surface could change the physicochemical properties of the ore-forming fluid and resulted in the precipitation of Mo and other metal elements.