To address the unclear mechanical response mechanisms of the cement sheath in deep oil and gas well cementing under complex temperature-pressure conditions，this study systematically investigated the coupled effects of curing temperature，curing pressure，testing temperature，and confining pressure on the mechanical properties of Class G oil-well cement. Specimens were prepared at different curing temperatures ranging from 25°C to 140°C and curing pressures of the atmospheric pressure and 21.7 MPa. Triaxial compression tests were then conducted at different testing temperatures from 25°C to 140°C and confining pressures from 0 to 20 MPa to analyze the evolution of stress-strain curves，peak stresses，and elastic moduli. The results show that increasing curing temperature reduces the peak stress and elastic modulus of the set cement，while high-pressure curing significantly inhibits pore development and improves structural compactness. Increasing testing temperature helps enhance the mechanical performance of cement at the testing temperature close to its curing temperature. Increasing confining pressure improves the plasticity and residual strength of cement，and the cement exhibits staged evolutionary characteristics under different coupled conditions. The findings of this study provide a theoretical basis and experimental support for the design of cement slurry systems and the integrity evaluation of cement sheaths in deep formations.