Abstract: In this paper, the formation age and petrogenesis of the monzonitic granites were confirmed through LA-ICP-MS zircon U-Pb dating and geochemical analysis in southern Xiao Hinggan Mountains.It is found that the isotopic age of monzogranite is 188±2Ma, suggesting Early Jurassic. The geochemical characteristics show that Si and ALK are abundant, CaO, Fe₂O₃, TiO₂, MnO, MgO and P₂O₅ are relatively poor, with A/CNK being 0.95~1.12 and A/NK being 1.17~1.32, implying aluminum-weakly peraluminous high potassium calc-alkaline series granites; They are enriched in large ion lithophile elements Rb, K and high field strength elements Hf, Zr, Th, and depleted in large ion lithophile elements Ba, Sr and high field strength elements Nb, Ta, Ti and P.The total rare earth elements (∑REE) are abundant, the distribution curve shows that the light rare earth elements are more abundant than the heavy rare earth elements and show right-inclined pattern, with weak negative Eu anomalies.The geochemical characteristics of elements indicate that the granites belong to I-type granites.Combined with regional research data, it is suggested that the Early Jurassic monzonitic granites in southern Xiao Hinggan Mountains were formed under an extensional environment similar to back-arc basin which might have been related to double-subductions of the Paleo-Pacific plate beneath the Eurasian continent and the Mongol-Okhotsk oceanic plate beneath the Ergima Massif, and the magmas originated from partial melting of lower crustal materials.