The Sanjiang Tethyan domain in SE Asia is one of the most important mineral belts in China. The Damajianshan (DMJS) W-Cu-As polymetallic deposit is located in the southern part of Sanjiang Tethyan domain, related to Triassic quartz porphyry. Detailed exploration thereafter shows that the reserves of W, Cu, and As are 0.09, 0.42, and 0.12Mt, respectively. The W is a typical oxyphile element and always coexisted with Sn-Li-Be-Nb-Ta. The Cu-As are typical sulphophile elements and always coexisted with Au-Ag-Pb-Zn-Sb-Hg. It is rarely reported that a deposit preserves tungsten and copper-arsenic simultaneously in the southern Sanjiang region. In this study, the S-Pb isotopic ratios of sulphides, the H-O isotopic compositions of fluid inclusions in hydrothermal quartz, and the He-Ar isotopic ratios of fluid inclusions in sulphides were analysed to constrain the origin of the DMJS deposit. Studies have shown that the S-34 values of sulphides from ores are mainly between -6.17 parts per thousand and +0.02 parts per thousand; the sulphur isotope compositions indicate that the ore-forming materials might originate from deep sources. The Pb isotope characteristics indicate that the ore metals might originate from deep sources and be closely related to the quartz porphyry. The O-18(fluid) values of ore-forming fluids calculated from hydrothermal quartz range from -5.5 parts per thousand to +7.5 parts per thousand, and the D values of the fluid inclusions in quartz are from -81.7 parts per thousand to -50.2 parts per thousand. The H-O isotope systematics indicate that the ore fluids in the DMJS deposit were probably initially sourced from magmatic water and later gradually mixed with Mesozoic meteoric water. Fluid inclusions in pyrite, chalcopyrite, arsenopyrite, and pyrrhotite possess He-3/He-4 ratios of 0.02-0.05Ra, and their Ar-40/Ar-36 ratios range from 562.47 to 4159.15, indicating a complete crustal fluids and with higher radiogenic Ar-40. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore-forming fluids have a deep source. The unique geologic structure of the Sanjiang Tethys tectonic metallogenic region and the evolution of ore-forming fluids both promoted the formation of this rare DMJS deposit.