The invention provides a method for replacing filter cores in a fly ash filter. The method comprises the following steps: dismounting Venturi of all groups of filter devices, and sending out the Venturi from a first manhole to obtain dismounted Venturi; dismounting the filter cores of all the groups of filter devices, and sending out the filter cores from a second manhole; feeding 24 groups of new filter cores from the second manhole, and re-mounting the 24 groups of new filter cores; feeding the dismounted Venturi from the first manhole, and re-arranging the dismounted Venturi above all the groups of the new filter cores; and inspecting until qualification, and sealing the first manhole and the second manhole in order to complete replacement of the filter cores in the fly ash filter. The method for replacing the filter cores in the fly ash filter has the advantages of short working time, low safety risk and low construction cost.