The invention relates to a low-temperature SCR (selective catalytic reduction) denitration reactor with internal pre-oxidation unit and a method for denitration by the same.Flue gas is divided into two streams at a flue gas inlet, one stream of the flue gas directly enters the reactor while the other stream of the flue gas enters a pre-oxidation compartment, NO in the flue gas is oxidized into NO2 under the action of a catalytic oxidation layer, the pre-oxidized flue gas is mixed with the unoxidized flue gas, and a mole ratio of NO to NO2 in the mixed flue gas is (0.8-1.15):1.The mixed flue gas is well mixed with NH3 sprayed by a nozzle through a static mixer, and a low-temperature SCR reaction is carried out on the catalytic oxidation layer, wherein the internal temperature of the reactor is 200-350 DEG C.The low-temperature SCR reactor has the advantages that a main reaction in the reactor is changed into the rapid SCR reaction through the internal pre-oxidation unit, the reaction rate is more than ten times of a standard SCR reaction rate, denitration efficiency of a low-temperature flue gas section is greatly improved, and denitration efficiency is up to 98%.