Abstract: To investigate the effect of free-phase volume on gas adsorption capacity, adsorption measurements were carried out using the volumetric method. The influence characteristics of free-phase volume on gas adsorption capacity and adsorption constants were analyzed, and a correction model for gas adsorption constants was established. The results show that significant differences exist between the real free-phase volume and the apparent free-phase volume under high pressure. The free-phase volume has a remarkable influence on the gas adsorption capacity and adsorption constants of coal with strong adsorption ability. Under the same coal sample conditions, the deviation of the real free-phase volume increases with rising pressure. For different coal samples under the same pressure, a stronger adsorption capacity leads to a larger deviation in the real free-phase volume. The gas adsorption capacity increases by a maximum of 0.8640 cm³/g for every 1 cm³ reduction in free-phase volume, meaning that the gas adsorption capacity increases by up to 0.86% for each 1% decrease in free-phase volume. Using the real free-phase volume to calculate gas adsorption capacity can effectively avoid the occurrence of the “negative adsorption” phenomenon and the downward trend of adsorption isotherms under high pressure. Compared with the apparent gas adsorption constants, the real gas adsorption constant a is larger and b is smaller. The correction model for gas adsorption constants established based on the deviation shows high reliability and wide applicability, and can significantly improve testing accuracy and efficiency.