The cement sheath is a structure formed by filling cement slurry between the casing and the formation and letting it solidify and harden. It serves to fix the casing and seal the gap between the casing and the formation. Together with the casing and open hole section, it constitutes the wellbore. As the only channel for gas injection and withdrawal in the UGS, the wellbore is subject to loads such as high-frequency gas injection and withdrawal, in-situ stress, and temperature, and is a weak part of the UGS safety. During the gas injection, the casing, cement sheath, and formation are compressed by the internal pressure of the gas. This results in the cement sheath undergoing significant compressive strain. During the gas withdrawal, the radial stress on the casing, cement sheath, and formation decreases. The compressive strain is released, and tensile strain may even occur locally. Under this alternating load, the cement sheath, as a type of rock, will undergo plastic deformation, which leads to damage and failure of the cement sheath. Based on the above background and objectives, this chapter mainly presents the following research work: (1) Plastic deformation and damage of cement sheath; (2) Mechanical evolution of cement sheath under high-frequency gas injection and withdrawal; (3) Characterization and engineering application of cement engineering mechanics characteristics.