Abstract: Sealing technology is crucial in developing compressed air energy storage (CAES) man-made cavern power plants, and the selection of sealing materials is paramount. To verify the feasibility of polyurethane polymer mortar (PPM) as a sealing material for CAES man-made cavern, experiments on gas permeability and mechanical properties are conducted on PPM. Additionally, numerical simulations using FLAC^3D are performed to analyze the PPM sealing layer's structural stress characteristics and leakage rate. The results indicate that PPM has excellent gas-tightness performance, with gas permeability on the order of 10^-20~10^-22 m², which satisfies the sealing requirements of the CAES sealing layer. PPM has excellent tensile properties, low elastic modulus, high deformation capacity, and self-bonding ability with concrete. When the PPM intrinsic permeability is 10^-19 m², the leakage rate of man-made cavern is 0.215%, which meets the permissible air quality leakage rate requirement within one day. Due to the low elastic modulus of PPM, the calculated hoop stresses of the PPM sealing layer are all compressive stresses, and the maximum hoop tensile strain is only 1.15%~1.20%, which is much smaller than its ultimate tensile strain, thus avoiding tensile failure of the sealing layer. Therefore, PPM can meet the requirements of CAES sealing material in terms of gas permeability and mechanical indices.