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Abstract: Coalbed methane (CBM) resources have enormous potential in energy exploration and development, and evaluating the gas content in CBM is crucial for resource assessment and development. As the P-wave of coal is sensitive to changes in gas content, the mechanisms trough which adsorbed and free gas influence the P-wave amplitude are investigated, and a total gas content model based on the attenuation coefficient of the P-wave amplitude is established. For this purpose, the P-wave amplitude and gas content of coal are measured by using nonadsorbed gas (helium) and adsorbed gas (methane) in a special ultrasonic–isothermal adsorption experimental setup at various gas pressures. The experimental results demonstrate that the P-wave amplitude decreases logarithmically with increasing pore pressure and that free gas mostly affects the effective stress of coal by changing the squirt flow in the coal pore–fracture system. Adsorbed gas reduces the porosity of coal by adsorption-induced swelling, which is the primary mechanism underlying the increase in P-wave amplitude. When a coal seam contains both adsorbed and free gas, at low pore pressures (low total gas content), the effect of adsorption-induced swelling on the P-wave amplitude is the dominant factor, whereas the impact of effective stress becomes dominant at high pore pressures (high total gas content). The established model describes the mechanisms through which gas coexistence in different coal seam occurrence states impact the P-wave amplitude without introducing any assumptions. The fitting results strongly agree with the experimental data points; R2 is greater than 0.90, which reflects the rationality and applicability of the model and the method to determine the model parameters. This study reveals the mechanisms underlying the changes in geophysical parameters caused by adsorbed and free gas, which can provide scientific guidance for the evaluation of gas content in deep CBM reservoirs.