Radio frequency (RF) heating is a novel thermal extraction method in the development of coal underground in-situ transformation. In the heating process, coal seam is usually in the state of gas-water coexistence. To explore the impact of moisture content on RF heating of coal seams, this study carried out a series of experiments based on the underground RF heating experimental system. The experimental results show that as the moisture content increases, the heating rate of the coal seam first increases and then decreases. This is because the presence of moisture leads to an increase in the effective relative dielectric constant and effective specific heat capacity within the coal seam. Increasing the moisture content of coal seam in a moderate range can effectively improve the energy absorption and heat exchange efficiency of coal seam. However, since the specific heat capacity of water is quite large, heating the same volume of coal sample will require more heat energy, so the heating rate will decrease accordingly. Moreover, coal seams with high moisture content will produce more water vapor during RF heating, which may interfere with the pyrolysis and gasification reactions of coal seams. Therefore, reasonable control of the moisture content of the coal seam is crucial in coal seam RF heating technology. This study not only reveals the mechanism of the influence of moisture content on the RF heating effect of coal seams but also provides theoretical support and practical guidance for optimizing the radio frequency heating process.