To accurately and quantitatively characterize the evolution of coal microstructures before and after supercritical CO2 treatment, this paper proposes a microstructural analysis for coal-rock based on low-field nuclear magnetic resonance (LNMR) technology, the L-weighing method. The experimental results of the L-weighing method and the LNMR method were compared and analyzed. The results showed that there were differences between the experimental results of the L-weighing method and the LNMR method, but the overall laws were basically the same. The L-weighing method quantitatively characterizes the water content in coal and determines water consumption and microscopic component extraction in coal with supercritical CO2, while the LNMR method only determines the dissolution of H-containing substances in the coal by supercritical CO2. According to the combined analysis with the L-weighing method and the LNMR method, after the supercritical CO2 entered the coal, it consumed 0.33 g of water from the coal and extracted 0.32 g of microscopic components from the coal, reacted chemically with both water and microscopic components in the coal, and provided effective water consumption and extraction of the coal. The supercritical CO2 increased the saturable water in the coal by 0.71 g, the porosity by 1.57%, and the pore seepage volume by a factor of 1.10, which expanded the volume and increased the permeability of the coal. After the reaction with supercritical CO2, the centrifugal water loss of the coal increased by 0.16 g, the T2 cutoff (T2C) of the coal had shifted to the left by 0.31 ms, and the free water volume of the coal had increased by a factor of 1.15, which enhanced the pore connectivity in the coal. The water consumption, extraction, volume expansion, permeability increase, and pore connectivity increase were primarily responsible for the changes in the coal microstructure effected by supercritical CO2.