The Huangshan-Jingerquan belt in Northwest China is one of the most important orogen-hosted magmatic Ni-Cu sulfide metallogenic belts worldwide. The Huangshandong, Huangshan, and Tulaergen deposits are the three largest Ni-Cu deposits along the belt. The orebodies are situated inside mafic-ultramafic complexes. Sulfur isotope data and trace element composition of the sulfide ores and country rocks are used to evaluate the importance of crustal S addition for sulfide mineralization and speculate as to the source of the sulfur that triggered sulfide segregation. The S/Se ratios of >95% of the sulfide ores range from 2,398 to 85,222—higher than that of the mantle (2,850–4,350) but lower than the country rocks (S/Se = 3,889–160,769). The sulfide ores from the Huangshandong, Huangshan, and Tulaergen deposits have restricted δ34S values ranging from –0.86 to 1.33, 0.26 to 0.75, and –0.2 to 1.4‰, respectively. However, the country rocks of these mafic-ultramafic complexes have highly heterogeneous δ34S, ranging from –22.3 to 18.8, –22.3 to 2.12, and –1.4 to 5.3‰, respectively. Arsenic/bismuth and Sb/Bi ratios of the sulfide ores range from 0.22 to 7.59 and 0.02 to 2.88, respectively, which are lower than those of the country rocks (3.17–243 and 2.8–33) and mid-ocean ridge basalt (MORB) (5.09–127 and 0.51–9.25). The values of δ34S and S/Se as well as ratios of As/Bi and Sb/Bi of the sulfide ores indicate that the sulfide segregation and consequently the formation of the deposits were closely associated with the addition of crustal sulfur, whereas the sulfides in the country rocks have not been incorporated into the mineralization. Thus, it is proposed that assimilation of external crustal sulfur at depth might play a critical role in triggering sulfide immiscibility and the formation of the magmatic Ni-Cu deposits in the Huangshan-Jingerquan belt.