Abstract: This article reports the microscopic properties of polyurethane and expansive cement jelly composite material (PD composite material) as well as their impacts on the sealing performance of boreholes. Simulated experiment of the coal mine drilling sealing process was carried out, and FEI Quanta^TM 250 environmental scanning electron microscopy was used for analyzing the microstructure of polyurethane and PD composite material, together with the combination, penetration and development of these two materials with the coal wall. It is found that the permeability coefficient of PD composite material is approximately 1/48 of that of polyurethane. Polyurethane has a cellular network structure and larger internal porosity, and there exists a blank area between polyurethane and the hole wall. However, PD composite material has a smaller internal porosity, and the tight structure makes the combination of PD composite material with the hole wall more compact. Compared with polyurethane, PD composite material is easier to overcome gas pressure and water resistance and then gradually infiltrate at the fracture zone around the borehole. Moreover, PD composite material can continue to develop in residual cracks and holes around the borehole.