A reducing agent pyrolysis system for a selective catalytic reduction apparatus, the reducing agent pyrolysis system comprising: an elbow duct which is suitable to be installed on an exhaust duct at a front end of a reactor, allows exhaust gas to flow into the elbow duct, and allows the inflow exhaust gas to be discharged toward the reactor; an inner pipe unit which is disposed in the elbow duct, and allows a part of the exhaust gas to flow into and be discharged from the inner pipe unit; a heating device which is installed in the inner pipe unit, and heats the exhaust gas flowing into the inner pipe unit; and a nozzle which is installed in the inner pipe unit, and disposed at a rear end of the heating device based on a flow of the exhaust gas, and adapted to inject a reducing agent into the inner pipe unit, characterised in that, (i) the elbow duct has a first main pipe and a second main pipe which are disposed and connected at a right angle or an obtuse angle, and the first and second main pipes and have a greater size than the exhaust duct, wherein the reducing agent pyrolysis system further comprises an orifice unit which is positioned in the first main pipe, wherein a hole is formed in the orifice unit, and an end of the inner pipe unit, which is positioned at a portion where the exhaust gas flows in, is disposed in the hole and/or (ii) the reducing agent pyrolysis system further comprises a valve unit which is installed at a portion of the inner pipe unit where the exhaust gas flows in, wherein a flow rate of the exhaust gas flowing into the inner pipe unit is controlled by controlling an opening and closing operation of the valve unit. an elbow duct which is suitable to be installed on an exhaust duct at a front end of a reactor, allows exhaust gas to flow into the elbow duct, and allows the inflow exhaust gas to be discharged toward the reactor; an inner pipe unit which is disposed in the elbow duct, and allows a part of the exhaust gas to flow into and be discharged from the inner pipe unit; a heating device which is installed in the inner pipe unit, and heats the exhaust gas flowing into the inner pipe unit; and a nozzle which is installed in the inner pipe unit, and disposed at a rear end of the heating device based on a flow of the exhaust gas, and adapted to inject a reducing agent into the inner pipe unit, characterised in that, (i) the elbow duct has a first main pipe and a second main pipe which are disposed and connected at a right angle or an obtuse angle, and the first and second main pipes and have a greater size than the exhaust duct, wherein the reducing agent pyrolysis system further comprises an orifice unit which is positioned in the first main pipe, wherein a hole is formed in the orifice unit, and an end of the inner pipe unit, which is positioned at a portion where the exhaust gas flows in, is disposed in the hole and/or (ii) the reducing agent pyrolysis system further comprises a valve unit which is installed at a portion of the inner pipe unit where the exhaust gas flows in, wherein a flow rate of the exhaust gas flowing into the inner pipe unit is controlled by controlling an opening and closing operation of the valve unit. (i) the elbow duct has a first main pipe and a second main pipe which are disposed and connected at a right angle or an obtuse angle, and the first and second main pipes and have a greater size than the exhaust duct, wherein the reducing agent pyrolysis system further comprises an orifice unit which is positioned in the first main pipe, wherein a hole is formed in the orifice unit, and an end of the inner pipe unit, which is positioned at a portion where the exhaust gas flows in, is disposed in the hole and/or (ii) the reducing agent pyrolysis system further comprises a valve unit which is installed at a portion of the inner pipe unit where the exhaust gas flows in, wherein a flow rate of the exhaust gas flowing into the inner pipe unit is controlled by controlling an opening and closing operation of the valve unit.