The present invention provides a method of recycling a spent flue gas denitration catalyst and a method of determining a washing time of the spent flue gas denitration catalyst. The method of recycling the spent flue gas denitration catalyst includes physically removing solids deposited in the spent flue gas denitration catalyst, removing poisoning substances deposited in the spent flue gas denitration catalyst by washing the spent flue gas denitration catalyst with a washing liquid for a washing time determined by measuring the hydrogen ion concentration of the washing liquid and drying the resulting spent flue gas denitration catalyst.