The invention relates to a method for preparing SCR (Selective Catalytic Reduction) denitration catalyst by slag and a catalyst thereof. The catalyst uses slag remained after potassium manganate is extracted as a main raw material. The preparation method comprises the following steps of: washing slag by a nitric acid solution, filtering the slag, drying and pulverizing; fully mixing filter slag powder with active carbon powder; then after adding titanium dioxide sol and deionized water for being adjusted into proper slurry, rubbing into strips and preparing small balls; and drying and roasting in sequence for activating. Compared with the prior art, the SCR denitration catalyst has catalytic activity in the range of smoke temperature of 150-350 DEG C and higher catalytic activity compared with the prior art within 200 DEG C and 300 DEG C (particularly 250 DEG C or so); and by applying the catalyst of the invention for carrying out SCR denitration, the denitration rate is high. The invention also has the advantages of raw material cost saving, elimination of pollution of high-valence manganese-contained wastes on environment and simple preparation process.