The Baguamiao gold deposit discovered in the early 1980s is located in the northern part of Pb- Zn multi- metal ore- field in the Fengtai basin, Qinling orogen. It is one of the largest gold deposits in Shaanxi Province, with proven Au reserves of about 106 t. Its dominant country rock is low- grade metamorphosed muddy clastic rock and carbonate rocks of the Upper Devonian Xinghongpu Formation. The ore- forming process of the Baguamiao gold deposit was composed of three main ore- forming periods, including (a) strata- bounded pyrrhotite- quartz vein period, (b) NE- oriented pyrite- quartz vein period, and (c) fracture- hosted sulfide- calcite vein period. The NE- oriented pyrite- quartz vein period were further divided into four ore- forming stages based on mineralogical characteristics, including euhedral pyrrhotite- pyrite- coarse- grained quartz stage (Ⅰ), anhedral pyrite- electrum- fine- grained quartz stage (Ⅱ), anhedral pyrrhotite- native gold- calcite stage (Ⅲ), and biotite stage (Ⅳ). In- situ S isotope analyses of sulfide in country rock and NE- oriented pyrite- quartz veins were carried out using the LA- MC- ICP- MS, in order to constrain the ore- forming material source and mineralization mechanism of NE- oriented pyrite- quartz veins. The results show that the δ 34 S values of pyrrhotite in phyllite without alteration are concentrated in 11.6‰~13.0‰, which lie between the δ 34 S data of original pyrite from 3.3‰ to 16.0‰ in previous publication. δ 34 S data of stage I euhedral pyrite are from 8.4‰ to 10.1‰, and those for euhedral pyrrhotite are from 7.6‰ to 8. 0‰; δ 34 S values of stage Ⅱ anhedral pyrite are much higher, from 14.0‰ to 15.9‰; while those of stage Ⅲ pyrrhotite are from 6.4‰ to 8.3‰. The δ 34 S values of sulfide in the NE- oriented pyrite- quartz veins have large dispersion and enriched heavy sulfur isotope, and the values of sulfide in different stages are between that of granite in ore concentrated area and the wall rock, indicating that the sulfur source is generally characterized by the mixing of stratum sulfur and magmatic sulfur. Combined with the mineral association (pyrrhotite+pyrite, pyrite, pyrrhotite) from stage Ⅰ to Ⅲ, we propose that the increase of  δ 34 S value from stage Ⅰ to Ⅱ is caused by sulphidation generated by fluid- wall rock interaction, while the decrease of δ 34 S in stage Ⅲ is related to the mixing of magmatic water into ore- forming system. Based on the physic- chemical conditions of NE- oriented pyrite- quartz veins, the transportation of Au in ore- forming fluid is constrained to be Au (HS)-2. Sulphidation caused by the fluid- rock reaction from stage Ⅰ to stage Ⅱ induced the gold precipitation in stage Ⅱ, and mixing of magmatic water in the stage Ⅲ is the main mineralization mechanism in stage Ⅲ.