Hydraulic coupling is an important factor causing the instability of surrounding rock and water inrush disaster. The hydraulic coupling tests were conducted by using TAW-2000 rock mechanics testing system on the cretaceous weakly cemented red sandstone, the nonlinear mechanical behaviors and strength characteristics were analyzed under different seepage pressures and confining pressures, and the degradation mechanism of seepage pressure on red sandstone was explored. Combined with the effective stress principle, D-P criterion and strain equivalence theory, a damage constitutive model reflecting the hydraulic coupling effect was established. The results show that: Confining pressure restricts crack propagation and enhances mechanical properties. As confining pressure increases, the strength and elastic modulus of red sandstone increase, while the Poisson's ratio decreases; As the pore water pressure increases, the strength and elastic modulus of red sandstone decrease, and the Poisson's ratio increases, exhibiting a significant softening effect. Red sandstone has low clay mineral content and weak cementation ability, under the action of water pressure, the cement between particles dissolves and loses the cementation strength, resulting in a significant decrease in cohesion, the loss of cementation strength is the internal reason for the softening of red sandstone. The constitutive model established is suitable for conventional triaxial test and hydraulic coupling test, and can better describe the stress-strain relationship and the law of damage evolution of red sandstone. With the increase of parameter m, the plastic deformation ability of rock decreases, and with the increase of parameter F₀, the rock resistance to damage and deformation increases.