The application relates to the technical field of auxiliary feedwater for a 1000 megawatt nuclear power plant, in particular relates to a cooling system and method for a auxiliary feedwater system water storage tank of the 1000 megawatt nuclear power plant. The cooling system comprises a deaerator circulating pump and a heat exchanger. The water outlet of the auxiliary feedwater system ASG water storage tank is connected with the water inlet of the deaerator circulating pump, the water outlet of the deaerator circulating pump is connected with the water inlet of the heat exchanger, and the water outlet of the heat exchanger is connected with the water inlet of the ASG water storage tank to form a circulating cooling loop. The deaerator circulating pump is used for providing circulating power to inject water in the ASG water storage tank into the heat exchanger. The heat exchanger is used for cooling inflow water and outputting cooled water to the ASG water storage tank. By adopting the heat exchanger to carry out cooling treatment, the water temperature of the ASG water storage tank can be ensured to be within the technical specification, so that alarms caused by the water temperature of the ASG water storage tank exceeding the upper temperature limit can be avoided, and further the unit can be prevented from retreating to the normal shutdown mode of waste heat removal/cooling.