Abstract: Optimal theories were applied to scheme design of nuclear power plant, and the nuclear power plant weight served as a performance evaluation index, the weight evaluation models of main equipments in primary circuit were established, based on the sensitivity analysis of thermal parameters which influence weight of nuclear equipments in primary circuit The optimal design of primary circuit weight was carried out using an improved complex method algorithm The optimal results indicate that the weight of optimum design is 3.77% less than original design, which proves the possibility of optimization and can provide reference for engineering design