The coal gangue after grinding was added into concrete instead of part of cement, and the coal gangue foam concrete was prepared by physical foaming process. Then, the pore size distribution, average pore size and pore diameter of foam concrete were characterized by FiJi-imageJ image analysis technology. The reasons for improving the porosity of foam concrete by coal gangue and the effect of coal gangue particle size on the compressive strength of foam concrete were analyzed. The results show that when the coal gangue powder with particle size of 45 μm is added, the strength of foam concrete decreases with the increase of coal gangue content, while when the coal gangue powder with particle size of 15 μm is added, the strength increases first and then decreases with the increase of coal gangue content. The coal gangue powder with particle size of 45 μm is better than the coal gangue powder with particle size of 15 μm in adjusting the pore structure, and the optimal effect is achieved at the content of 50% (mass fraction).