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Gas reservoirs with high CO₂ have been found in several wells in the Hailar Basin. In this paper, a composition analysis, stable carbon isotope analysis, and a rare gas helium isotope ³He/⁴He and argon isotope ⁴⁰Ar/³⁶Ar analysis were carried out. These comprehensive analyses show that the CO₂ in the Hailar Basin is inorganic-origin gas, which generally has the characteristics of crust–mantle-mixed CO₂, and the fraction of helium of mantle source can reach 15.12~18.76%. There are various types of CO₂ gas reservoirs. CO₂ gas mainly comes from deep crust. The distribution of gas reservoirs is mainly controlled by deep faults and volcanic rocks, as well as by reservoir properties and preservation conditions. Magmatic rocks provide gas source conditions for the formation of inorganic CO₂ reservoirs. Deep–large faults provide the main migration channels for CO₂ gas. The sandy conglomerate and bedrock weathering crust of the Nantun Formation and the Tongbomiao Formation provide favorable reservoir spaces for the formation of CO₂ gas reservoirs. The combination of volcanic rock mass and deep–large faults creates a favorable area for CO₂ gas accumulation. The age of magmatic intrusion and the homogenization temperature of oil–gas inclusions in Dawsonite-bearing sandstone indicate that 120 Ma in the Early Cretaceous was the initial gas generation period of the CO₂ reservoir and that oil and gas were injected into the reservoir in large quantities in 122~88 Ma. This period is the peak period of magmatic activity in Northeast China, as well as when the crust of Northeast China greatly changed. A large-scale CO₂ injection period occurred in 100~80 Ma, slightly later than the large-scale injection period of the oil and gas. Since the Cenozoic, the structure has been reversed, and the gas reservoir has been adjusted.