Rapid encroachment of edge and bottom water in ultra-deep gas reservoirs often leads to water breakthrough in gas wells and productivity decline, which has become a major engineering issue restricting the long-term stable development of the Keshen Block in the Tarim Basin. Reservoir pressure depletion caused by continuous gas production and local pore-pressure redistribution induced by water encroachment alter the effective stress state of the reservoir, thereby causing rock deformation and the evolution of mechanical parameters. However, the coupled evolution of the effective stress coefficient b, elastic parameters, and mechanical parameters under high in situ stress and high pore pressure remains unclear. Therefore, effective-stress-coefficient measurements and triaxial compression tests were conducted on Bashijiqike Formation outcrop sandstone from the Kuqa Depression, Tarim Basin, under combined high in situ stress and high pore pressure conditions representative of 6000–8000 m burial; the evolution of the effective stress coefficient, Young’s modulus, and Poisson’s ratio was jointly characterized, and a unified stress-dependent framework linking the three parameters was proposed. The results show that, with increasing in situ stress, the effective stress coefficient and Poisson’s ratio decrease, whereas the elastic modulus increases. Increasing pore pressure produces the opposite trends, and the pore-pressure sensitivity of each parameter weakens with increasing in situ stress. Under a given in situ stress, the elastic modulus and Poisson’s ratio exhibit linear relationships with pore pressure. The effective stress coefficient b decreases nonlinearly with increasing simple effective stress and stabilizes at 0.67–0.68 when the simple effective stress exceeds 120 MPa. After incorporating the stress-dependent b, the elastic modulus and Poisson’s ratio can be uniformly expressed as functions of effective stress, exhibiting a sigmoidal saturating increase and an exponential decay, respectively. Validation using Well Keshen 2-1-14 shows that the average relative errors between the calculated and log-interpreted values are 7.42% for the elastic modulus and 12.38% for Poisson’s ratio.