The Xiarihamu Ni deposit is located in the East Kunlun orogenic belt in the northern part of the Qinghai-Tibet plateau. The regional tectonic setting, lithology, and mineralogy suggest that the Xiarihamu Ni deposit formed in a postcollisional setting. We use a slab break-off model to explain the simultaneous appearance of the Xiarihamu magmatic Ni sulfide deposit and the high-pressure eclogite exhumation in the East Kunlun orogenic belt. In situ δ534SV-CDT value of pyrite from the country rock (granitic gneiss) of the Xiarihamu complex is 11.2‰, whereas in situ δ534SV-CDT value of sulfides from the Xiarihamu I complex range from 2.4 to 7.7‰, with an average of 4.5‰. Crustal sulfur most likely accounts for approximately 40 to 60% of the total sulfur content in the Xiarihamu magma. Our modeling shows that the Xiarihamu parental magma may have achieved sulfide saturation and experienced approximately 0.005% sulfide segregation before complex emplacement. Based on previous studies and the platinum group element data in this study, we believe that the Xiarihamu magma was sulfide saturated when olivine began to crystallize. The zones with rapidly declining olivine Fo and clinopyroxene Mg# values correspond well with the locations of orebodies, implying that the crystallization of olivine or clinopyroxene played a role in the sulfide saturation. The whole-rock Ni content of the peridotite is inversely related to the Cr# and FeO contents in spinel, and the whole-rock Ni content of the websterite is negatively correlated with orthopyroxene Mg#; thus, during the crystallization of spinel and orthopyroxene, more sulfide formed. Our calculation shows that the decrease in sulfur content at sulfide saturation (SCSS) during fractional crystallization depends mainly on a decrease in temperature, and the change in the melt composition is a secondary or minor factor. Although the contribution of fractional crystallization to total sulfur content may be only approximately 3.3 to 6.0%, an increasing concentration of sulfides were accumulated at the latest crystallization in the host rock because of the incompatibility of sulfur. Recognizing this phenomenon is important for ore exploration. We logged 11 drill cores meter by meter and prepared 110 thin sections from across these 11 drill cores. The lithofacies distribution and crosscutting phenomena have been clearly determined. Combined with systematic geochemical data on the cross section, this paper summarizes the mineral crystallization sequence in different locations of the Xiarihamu I mafic-ultramafic magma chamber and depicts a six-stage model for sulfide saturation during the consolidation process of the Xiarihamu complex.