Underground saline aquifer CO2 dissolution and storage is one of the important forms of underground carbon storage, which plays a significant role in improving the climate environment. However, there is still a lack of visualization studies on the pore-scale CO2 dissolution diffusion process. Therefore, this paper aims to utilize microfluidic experimental technology to investigate the influence of salinity, temperature, and pressure on the dissolution diffusion behavior of CO2 in water, and attempt to characterize the dissolution diffusion process using mathematical formulas. The differences in CO2 bubble precipitation between pure water and saline water during depressurization are also analyzed. Experimental results indicate that under the same pore size, the average diffusion rate of CO2 in pure water is 2.3 times that in saline water; as the pore size decreases, the dissolution diffusion capability of CO2 increases; and the diffusion rate generally shows an initial increase, followed by a decrease, and eventually reaches a steady trend over time, indicating that the mass transfer velocity at the interface between the gas and liquid phases is much higher than the diffusion rate of CO2 molecules in the liquid phase. During the pressure reduction process, the diameter of CO2 bubbles precipitated in pure water is smaller, mostly concentrated in the range of 0–90 μm. The diameter of bubbles precipitated in saline water is relatively uniform. The research findings of this paper provide experimental basis for studying the dissolution diffusion behavior of CO2 in underground porous media.