Abstract: Sodium-cooled fast reactors use closed assemblies, and flow zoning is an important means to achieve core outlet temperature flattening. The calculation amount of the traditional flow zoning optimization design method increases exponentially with the number of fuel assemblies, which is not suitable for solving large-scale problems. In this paper, an optimization model for flow zoning design was established, and a genetic algorithm based on the optimal individual preservation strategy was designed. The maximum fuel temperature limit and cladding temperature limit were used as the boundary conditions to search for the optimal flow zoning solution. The average outlet temperature of the active zone is the highest, and the total flow of the active zone reaches the minimum. It provides a new approach to solve the core flow zoning optimization design problem of large-scale sodium-cooled fast reactor.