The Xiaonanshan Cu- Ni sulfide deposit is located in Siziwangqi, Inner Mongolia and is associated with several regional mafic- ultramafic intrusions. Their genetic relationship and mineralization potentials are still questionable, which restricts the mineral prospecting of Cu- Ni sulfide deposits in this area. In this study, we investigated the Xiaonanshan, Tu’naobao and Baiyin’aobao intrusions in aspects of gechronology, petrology, and geochemistry to constrain their petrogenesis and ore- bearing evaluation. The results show that there are two types of mafic- ultramafic intrusions in Xiaonanshan area. The Xiaonanshan and Tu’naobao Cu- Ni sulfide- bearing intrusions consist of gabbro and pyroxenite, with mineral assemblages mainly of plagioclase and clinopyroxene, and minor orthopyroxene and olivine, as well as uralitization, talcum, and chlorite alteration. The Baiyin’aobao gabbro mineral assemblage is mainly composed of plagioclase and clinopyroxene, which have altered into chlorite and zoisite, and without Cu- Ni sulfide. The Baiyin’aobao intrusion has similar formation age of 277.2±7.3 Ma to the Xiaonanshan intrusion (272.7±2.9 Ma). The Xiaonanshan and Tu’naobao gabbro have Mg# values of 0.56～0.67, with an m/f ratio of 1.25～1.98. Moreover, their MgO contents show negative relationships with Ni and Co, their Ni contents have positive correlations with Cu/Zr values, and they exhibit right inclined rare earth elements (REE) patterns. The Baiyin’aobao gabbro has Mg# values of 0.50～0.51, with an m/f ratio of 0.97～1.02. This intrusion shows unobvious relationships between its MgO contents with Ni and Co, has Cu/Zr values close to 1, and has flat REE patterns. The geochemical data indicate that the Xiaonanshan and Tu’naobao intrusions were sourced from lithosphere mantle modified by fluids. Their primary magma is high- MgO tholetiitic basaltic magma, with 10%～20% crustal contamination during ascent. The participation of external sulfur and fractional crystallization resulted in sulfide segregation to form the sulfide deposit. The Baiyin’aobao gabbro is formed by the crystallization of derived magma, with its parent magma having unobvious metasomatism. During the emplacement, the magma contaminated 5% crustal materials. There is not any discovered orebodies by drillings.