The Yuanbaoshan Late Carboniferous granite is located in the Yuanbaoshan area, Ejin Banner, Inner Mongolia. Its rock mass characteristics are of great significance for the study of the properties of the Hongshishan ophiolite belt and the reconstruction of the tectonic evolution of the northern Beishan region. LA-ICP-MS zircon U-Pb isotopic dating shows that the ages of granodiorite, monzonitic granite, and trondhjemite in the Yuanbaoshan composite batholith are 317.1+3.1 Ma, 314.8+2.7 Ma and 313.7+2.3 Ma respectively, suggesting Late Carboniferous. The petrogeochemical analysis shows that the granites of the Yuanbaoshan composite batholith have high silicon (SiO₂=67.77%~73.00%), and rich sodium (K₂O/Na₂O<1) with A/CNK being 0.95~1.13, belonging to partially aluminous to weakly peraluminous,with the enrichment of LILEs (Rb, Ba, Th, K), depletion of HFSEs (Ta, Nb, P, Ti) and medium strength negative europium anomaly (δEu=0.46～0.65), similar to the petrogeochemical characteristics of island arc environment. The trondhjemite is different from the granodiorite and monozonitic granite; it is rich in silicon, has very weak negative europium anomaly (δEu=0.81~0.99), high Sr/ Y and La/Yb ratios, and falls into the adakite area on the Sr/Y-Y diagram. These differences suggest that they have different source characteristics. According to the regional geological data, the subduction of the Hongshishan oceanic plate under the study area during the early Late Carboniferous initiated large-scale magmatic activity and formed the Yuanbaoshan granite composite batholith with island arc affinity. The plagioclase granite with adakite-like characteristics originated from partial melting of the subductional oceanic plate, implying that the Hongshishan Ocean might have had a relatively young hot oceanic crust during the early Late Carboniferous period.