Abstract: Secondary circuit system is important part of marine nuclear power plant whose weight and dimension are key factors to influence the rational arrangement of nuclear power plant. The trend of using high power and rapid propelling speed of marine nuclear power plant has resulted in that the weight and volume of secondary circuit system increase further, which causes difficulties to the design and arrangement of the nuclear power equipment, and that the marine maneuverability is seriously influenced. The mathematic model of secondary circuit system was established, and the corresponding codes were also developed. The sensitivity of design parameters influencing the secondary circuit system weight was analyzed. Taking the weight minimization as the objective, the design optimization of the secondary circuit system was carried out with the hybrid particle swarm optimization (HPSO) algorithm under the restriction conditions. Study results show that the secondary circuit system weight is reduced by 7% with the optimization scheme. Finally, the optimization results were analyzed, and the direction to guide design optimization of the secondary circuit system was indicated.