To study the mechanical properties changes and deformation of the underground coal pillar dam body in the mine goaf in the underground working environment, the Lijiatun Coal Mine is taken as the research background, and the laboratory similar simulation and FLAC3D numerical simulation method are adopted. The coal samples from 3-1 coal seam of Lijiatun Coal Mine were processed into standard samples according to ISRM. The samples were subjected to water immersion treatment for a certain period of time, and the mechanical properties analysis experiments were carried out. Subsequently, numerical simulation technology was used to simulate the actual working environment of the coal pillar dam and analyze the mechanical properties and deformation of the coal pillar dam. The study shows that water has a softening effect on coal samples. With the increase of water immersion time, the unidirectional compressive strength and unidirectional tensile strength of coal samples have decreased, but there is no regular decreasing trend; under the combined action of the overburden pressure, the coal pillar immersed surface is prone to horizontal deformation, the deformation gradually increases with time, and the deformation near the bottom plate is the largest, so that the on-site coal pillar is more vulnerable to damage.