CO2 injection into deep saline aquifers is prone to viscous fingering phenomena, limiting storage efficiency. In this study, 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 were reduced by 82.17% and 58.33%, respectively. CO2 migration proportion in mesopores was decreased by 16.3% while it was increased in smallpores by 3.5% through co-injection, with intensified negative convection phenomena observed in micropores. H? concentration in produced fluids was 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 were 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.