To accomplish the strategic objectives of carbon peaking by 2030 and carbon neutrality by 2060, while fulfilling international commitments, China has significantly increased the approval speed and number of nuclear power unit constructions in recent years. The “HPR1000” reactor, serving as the primary nuclear power unit design in China, derives one of its pivotal competitive advantages in the market from the enhancement of its economic efficiency. Currently, the main reinforcement of the nuclear island plant in the “HPR1000” demonstration project adopts a standard design, with the design reference peak ground acceleration (PGA) set at 0.3 g. Nevertheless, for specific sites where the seismic peak acceleration is significantly lower than 0.3 g, the influence of seismic forces on the structural components’ internal forces and reinforcement requirements warrants further investigation. Based on the NX plant, this study utilizes the acceleration graded response spectrum method for detailed calculation and analysis. It systematically compares the effects on internal forces and reinforcement of different types of components under various load combinations and evaluates the impact of optimized reinforcement design on the overall economic efficiency of the civil engineering investment for the building at specific sites. The results demonstrate that as the input peak seismic acceleration decreases, particularly below 0.2 g, the internal force values under SL-2 conditions exhibit no significant advantages and essentially lose their controlling influence on the the plant’s main reinforcement design. Furthermore, in certain specialized areas, specific load conditions contribute significantly to internal force values, thereby exerting more pronounced control over the plant’s principal structural components.