The invention provides a nuclear power plant auxiliary cool water system and a design method thereof. The auxiliary cool water system comprises a main water drain line assembly, an auxiliary cool water line assembly connected in parallel with the main water drain line assembly, a water inlet line assembly disposed on a water inlet path of the auxiliary cool water line assembly, and a switching unit connecting the main water drain line assembly and the water inlet line assembly, wherein the switching unit is used for controllably switching the main water drain line assembly or the water inlet line assembly to provide cooling water for the auxiliary cooling water line assembly, conducting the auxiliary cool water line assembly and the main water drain line assembly to establish siphon, and completing the start-up of the auxiliary cool water line assembly through siphon. By reducing the equipment configuration, the invention reduces the initial investment of the project, reduces the operation and maintenance cost and saves the auxiliary cooling water pump auxiliary power consumption while satisfying the system function.