Provided are a method of treating coal ash by mixing it with water, in which the coal ash and water undergo a temperature difference therebetween while they are mixed; and a method of desulfurization in a coal combustion boiler system, which comprises mixing coal ash that has been separated from a ash collector in the system, with water on the condition that the two undergo a temperature difference therebetween while they are mixed, and circulating the resulting mixture that serves as a desulfurizing agent into the coal combustor in the system. The desulfurization capability of the desulfurizing agent used in the desulfurization method is higher than that of the desulfurizing agent obtained through hydration of coal ash with water or steam.