The invention relates to the technical field of boiler coking control and particularly discloses a method for preventing coking of a coal-fired boiler. The method is achieved through the following steps that kaolin is added to fire coal, and the weight percentage content of the kaolin is 1.5%-4%; a hydraulic ash removal device is installed in a combustion chamber of the boiler; and WLB hydraulic ash blowers are installed in the 17900 mm-elevation position of the combustion chamber of the boiler and the centers of the 37300 mm-elevation positions of the four sides of the combustion chamber correspondingly. After the method, for preventing coking of the coal-fired boiler, provided by the invention is adopted, the temperature of an outlet of the combustion chamber is controlled effectively, and a flame kernel moves downwards obviously. The temperature of the outlet of the combustion chamber is controlled, and effective hydraulic ash removal is also conducted, so that obvious coking phenomena do not occur to a water cooling wall tube, a slag coagulating pipe, a high temperature superheater and a low temperature superheater on the upper portion of the combustion chamber.