CO2 capture and storage (CCS) is an effective way to reduce greenhouse gas emissions. Density-driven background flow in some deep saline aquifers is of interest in the geoengineering field, which can enhance dissolution rates compared with the diffusion process. This study aimed to investigate the instability of dissolution-driven convection using high-resolution magnetic resonance imaging (MRI) technology under injection conditions. A detailed discussion on the instability of the displacement front is presented. As the Rayleigh number (Ra) increased, the onset time decreased, and the finger number density increased, while the finger growth velocity increased with increasing Pelet number (Pe). Moreover, increasing the injection rate is beneficial to the mass flux for the same value of Ra. The transverse dispersion coefficient (D-T)-Pe curve in the range of 2.34 < Pe < 33.09 agrees with previous studies. The results provide a theoretical basis for dissolution-driven convection in CCS projects.