Expansion agent cannot fully react due to the lack of sufficient moisture inside high-performance expansive concrete. Superabsorbent polymer (SAP) provides the expansion agent reaction with sufficient moisture, and also supports the internal curing of concrete. In this paper, the effects of SAP dosage on mechanical properties and pore structure of expansive concrete are studied via experimental approach. Test results demonstrate that the dosage of SAP as of 0.3% brings about a maximum improvement of mechanical properties of expansive concrete and a surging of concrete’s strength by 5.4%. A proper amount of SAP also improves the internal pore structure of expansive concrete and enhances the reaction between expansion agent and concrete’s compositions; and in the meantime, SAP shrinks the pores in concrete, and condenses concrete’s structure by stimulating cement’s secondary hydration. Nevertheless, the dosage of SAP exceeding 0.3% generates unfavorable pores inside the concrete, resulting in a decrease in strength.