Understanding the effect of gas pressure on coal pore structure and dynamic mechanical properties can better guide the accurate monitoring of stress and gas in gas-containing coal seams in coal mines and efficiently prevent and control coal/rock-gas composite dynamic hazards. In this study, the characterization of the pore structure of the coal body under different gas pressures and three-dimensional impact compression tests were carried out. The findings demonstrate that when the axial static load and confining pressure are fixed, the gas pressure determines the amount of gas adsorbed by the coal samples and its pore structure changes. The effect of gas pressure on the pore structure of the micropores is not obvious, but it has an obvious dilatation effect on the pore structure of the macropores. Within the range of conditions and gas pressures studied in this paper, gas-containing coals’ dynamic compressive strength and failure strain decrease linearly with increasing gas pressure. The average dynamic strength deterioration rate of gas-containing coals increases linearly with an increase of gas pressure, which plays a deteriorating role in the dynamic mechanical properties of coal bodies. When the gas pressure increases from 0.7 to 2.8 MPa, the radius of the macropores inside the gas-containing coal increases 0.63 times, and the increased pores and cracks produce a stress concentration effect around the pores and cracks and the shorter time required for instability damage of the coal samples to occur when subjected to dynamic loading. The research results improve the basic theory of gas-containing coal dynamics and provide a theoretical basis for the mine coal/rock-gas composite dynamics disaster.