<p>Investigating the mechanical properties and damage mechanism of water-saturated sandstone under loading is highly significant. Uniaxial compression tests were performed on porous sandstone in dry and saturated states. First, based on the matrix-void assumption, a statistical-damage-based constitutive model for porous sandstone considering the compaction stage was established by integrating the statistical damage mechanics theory and the generalized strain equivalence principle, and the calculation of parameters was discussed. Second, the model was compared with the test results under the dry state. Furthermore, the influence of the water–rock interaction on mechanical properties and parameters was analyzed. Combined with the sensitivity analysis of Weibull parameters, a statistical-damage-based constitutive model considering the water–rock interaction was derived. Subsequently, the models under the two states were compared and analyzed with the test results. Finally, the proposed model was compared with existing ones regarding assumptions, influencing factors, application scope, and parameter determination. Research reveals: (1) The saturated samples exhibit a notably weaker bearing capacity and reach the capacity limit earlier; (2) The favorable concordance between the experimental and the constitutive curves for the dried samples validates both the soundness of the statistical-damage-based constitutive model incorporating the compaction stage and the applicability of the corresponding damage evolution model in elucidating the damage mechanism; (3) The model curve in the saturated state can reasonably represent the mechanical behavior of the original samples considering the interaction; (4) Owing to the rock-water interactive effect, the damage evolution process of the saturated samples is notably advanced.</p>