Mafic granulites as veins or lens are commonly identified within Milan Rock Group and TTG gneiss in Annanba area of Aksay County, Gansu Province. The granulite is composed of plagioclase (Pl), orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and magnetite (Mt), which is characteristic of typical medium-low pressure granulite facies rocks. Ti, P, Nb, Ta, Th, Hf, Sr and REE are well related to Zr of mafic granulites from Annan area, which shows that these elements remained basically unchanged and kept stable during the metamorphic process. Geochemical analysis shows that the mafic granulites belong to tholeiite series rocks, depleted in SiO₂, TiO₂, Al₂O₃ and P₂O₅, and enriched in CaO, MgO, Mg^# (41.52~43.09). The values of Fe₂O₃^T, MgO and CaO are negatively correlated with SiO₂, suggesting separation crystallization of pyroxene and amphibole during the evolution of the magma. The chondrite-normalized REE patterns show right-inclined feature with no obvious Eu(Eu/Eu^*=0.91~1.01) anomalies, whereas the primitive mantle normalized trace element spidergrams are characterized by enriched LILE (such as Rb, Ba and S) and depleted HFSE (such as Nb, Ta, Zr and Ti), similar to the typical feature of Phanerozoic arc basalt. Sr, Nd, Pb isotopic compositions suggest that the protolith of mafic granulites was derived from enriched mantle, and underwent crustal contamination. Combined with the tectonic evolution of southeastern Tarim, the authors hold that the protolith of mafic granulites was formed in island arc, and related to a subduction environment. The formation process of mafic magma may be as follows:the mantle wedge was metasomatized by the fluid and simultaneously formed enriched mantle; back-arc extension took place under the mechanism of subduction, which triggered partial melting of enriched mantle, and then the mafic magma moved upward to the upper crust along the fissures to form gabbro or diabase veins; later, they were metamorphized into mafic granulite under the granulite facies metamorphism.