<p>In seasonal frozen regions, large temperature differences, repeated freeze–thaw cycles, and salt-freeze erosion exacerbate the formation and expansion of microcracks in concrete. To address this issue, this study proposes a synergistic curing strategy combining internal curing with superabsorbent polymers (SAP) and external curing with water-based epoxy coating (WEC) to optimize the mix proportion of concrete with combined internal–external curing for seasonal frozen regions. First, through mortar-level experiments and gray target decision analysis, the optimal parameter combinations for SAP and WEC were determined. Building on this, the influence patterns of SAP dosage, WEC spraying amount, and water-reducing admixture dosage on concrete workability, mechanical properties, shrinkage, and durability were thoroughly analyzed. Finally, the characteristics of hydration products in concrete under different curing methods were studied using scanning electron microscopy (SEM). The test results showed that when SAP had a particle size of 60–90 mesh with a dosage of 0.24%, and WEC had an emulsion-to-solid ratio of 20:4 with a spraying amount of 200 g/m², the mortar exhibited optimal mechanical properties and shrinkage performance. Based on this, the optimal material parameter combination for bridge concrete in seasonal frozen regions was determined as: SAP dosage of 0.24%, WEC spraying amount of 225 g/m², and water-reducing admixture dosage of 1.2%. SEM tests revealed that concrete with combined internal–external curing had a denser microstructure compared to standard-cured concrete. This study provides scientific guidance and reference value for enhancing the durability and engineering application of bridge concrete with combined internal–external curing in seasonal frozen regions.</p>