Abstract: To address the critical shortage of high-quality coking coal resources in China, meager lean coal was ?hy-dro-modified in a subcritical H?O-CO system to improve its caking properties. The impact of three inde-pendent variables—water-coal ratio, CO initial pressure, and reaction temperature—on the caking index (GRI) of modified coal was analyzed utilizing response surface methodology (RSM). A quadratic model was constructed to illustrate the relationship between these variables and the maximum caking index obtainable under optimal conditions. The model was validated through analysis of variance (ANOVA), contour analysis, and response surface analysis, which revealed the interactions among the variables. The microstructure and pyrolysis characteristics of both raw and modified coals were examined utilizing XRD, FTIR, 13C-NMR, and TG-DTG techniques. The results indicated that the water-coal ratio, CO initial pressure, and temperature significantly influenced the caking index of modified coal, with CO initial pressure exerting the most pronounced effect, followed by temperature and water-coal ratio. A significant interaction was noted between CO initial pres-sure and temperature. The predicted GRI value from RSM aligned closely with the experimental value under optimized conditions: a water-coal ratio of 1.58, CO initial pressure of 3.63 MPa, and a temperature of 350.8 ℃. The GRI value closely approximated the experimental value, with the lean coal’s GRI rising from 12 to 81.78, and the error between experimental and predicted values not exceeding 2%. The hy-dro-modification was shown to break chemical bonds, shorten the length of fatty chains in meager lean coal, thereby increasing its branchedness, and remove oxygen-containing functional groups in order to weaken the cross-linking structure and form stable protonated aromatic carbons (faH). The reduction in the size of the aromatic nuclei (Xb), along with the decrease in the bridged aromatic carbons (faB), suggested that some of the aromatic linkages were broken. As a result, the lamellae were re-stacked more tightly under high pressure, leading to a decrease in the crystal surface spacing (d002) and an increase in both the number of stacked layers (N) and stacking height (Lc). Collectively, these structural changes contributed to a significant enhancement in the caking properties of coal.