CO₂ injection into deep saline aquifers is prone to viscous fingering phenomena, limiting storage efficiency. Gas-brine co-injection displacement experiments were conducted using nuclear magnetic resonance (NMR) technology to systematically investigate CO₂ migration characteristics and enhancement mechanisms for storage capacity. The results demonstrate that gas-brine co-injection induced an intermittent flow pattern characterized by spontaneous separation and aggregation of CO₂ and brine, by which CO₂ and brine flow velocities are reduced by 82.17% and 58.33%, respectively. CO₂ migration proportion in mesopores is decreased by 16.3% while it is increased in smallpores by 3.5% through co-injection, with intensified negative convection phenomena observed in micropores. H⁺ concentration in produced fluids is significantly elevated by co-injection, with CO₂ dissolution reaction extent reaching 3.98 times that of single-phase injection at gas-water ratio (GWR) of 0.5. Distinct influence mechanisms under different GWR are revealed by relative permeability curves: residual trapping enhancement mechanism (GWR = 0.2), displacement-trapping balance mechanism (GWR = 0.5), and displacement-dominated weak trapping mechanism (GWR = 0.8). Based on comprehensive evaluation using normalized displacement efficiency and storage efficiency, it was indicated that CO₂ storage capacity can be effectively enhanced by gas-brine co-injection under conditions of 0.2 mL/min and GWR of 0.5. This research provides theoretical foundation and optimization strategies for CO₂ geological storage engineering in deep saline aquifers.