This paper selects the Early Cretaceous intrusive rocks in the Mount Nuomin area in the northern Great Hinggan Mountains to study the zircon U-Pb geochronology and geochemistry, and discuss the genetic types, magma sources and tectonic environment of the intrusive rocks in this area. The lithology of intrusive rocks in this area is mainly syenite, syenite granite porphyry and porphyritic syenite. LA-ICP-MS zircon U-Pb dating of syenite samples shows that the emplacement age of syenite is 129.5±0.4 Ma, which should be the product of early Cretaceous magmatic activity, and combined with the contact relationship between intrusive rocks and strata in the area, the formation age of intrusive rocks in this area is Early Cretaceous. The Intrusive rocks are characterized by high silica（SiO2=67.36%~74.09%）, alkali（K2O+Na2O=8.88%~9.34%） and alumina（Al2O3= 12.56%~16.15%），with low MgO, TiO2 and CaO. They are of high potassium calcium alkaline rock series and Quasialuminum—peraluminum as indicated by A/CNK ratio of 0.94~1.31. The trace elements are enriched in large ion lithophile elements such as Rb, U, Th, K, and strongly depleted of high field strength elements such as Ti, Nb, Sr, P, and have obvious negative Eu anomalies, which belong to highly differentiated type I granites. The rock Rb/Sr is 0.9~2.0, Sr/Y is 4.2~7.2, showing the characteristics of high Sr and low Y, indicating that the magma source area is the partial melting of crustal materials. Based on regional research results, the Mongolia—Okhotsk tectonic domain has ended its collision before the Early Cretaceous, and the early Cretaceous magmatic activity in the Mount Nuomin area may have occurred in the extensional environment after the Mongolia—Okhotsk orogen.