Abstract: The steam generator is one of the key equipments in the PWR nuclear power plant, which has relative large weight and dimension, nowadays, it tends to using high capacity steam generators, which makes the steam generators to be heavier and larger, and it causes difficulties to the transport and arrangement of steam generators. Therefore, it is necessary to apply optimization techniques to the design of the steam generator in order to achieve the minimum weight or volume by means of finding the optimum combination of design parameters while satisfying the heat exchange requirements and safety constraints. In this work, the evaluation model and corresponding codes of a vertical natural circulation steam generator were established, and based on the sensitivity analysis of thermohydraulic parameters and structural parameters which influence steam generator weight, the optimal design of steam generator, taking the net weight as objective, was carried out using a complex-genetic algorithm. The optimal results indicate that the net weight of optimum design is 17.16%, less than original design. The result shows an obvious effect which proves the possibility of optimization and can provide reference for real engineering design.