Abstract：The hydrothermal ore-forming process of the Nannihu-Sandaozhuang Mo (W) deposit can be divided into four stages, i.e., from early to late, (1) skarn and hornfels stage, (2) quartz - K-feldspar and retrograde stage, (3) quartz-sulfides stage and (4) quartz-carbonate stage. Based on studies of carbon, hydrogen and oxygen isotopes of different stages, the authors hold that the ore-forming fluid was derived mainly from the magmatic system and subordinately from the carbonate strata with minor organic matter. The atmospheric water probably made very little contribution to the ore-forming process. From early to late stage, the ore-forming process probably experienced immiscibility between melt and volatile-rich hydrothermal fluid, water-rock reaction, and phase separation of CO2 rich gas from ore fluid caused by sudden decompression.