A low-pressure SCR system, comprising: a SCR reactor; an exhaust line configured to guide a low-temperature exhaust gas passing through a turbocharger to the SCR reactor; a urea decomposing device having a urea decomposition chamber and configured to decompose urea to generate ammonia and send the generated ammonia to the SCR reactor; a first bypass line configured to extract a portion of a high-temperature exhaust gas before flowing from an engine into the turbocharger and guide the portion of the high-temperature exhaust gas to the urea decomposing device; a second bypass line configured to connect the first bypass line and the exhaust line to distribute the high-temperature exhaust gas flowing in the first bypass line to the exhaust line so that a flux of the bypass gas supplied to the urea decomposing device through the first bypass line is changed; characterised in that the system further comprises: a third bypass line configured to connect the exhaust line and the first bypass line to have a diverging point in the exhaust line at an upstream of a joining point of the second bypass line and have a joining point in the first bypass line at a downstream of a diverging point of the second bypass line, the third bypass line extracting a portion of the low-temperature exhaust gas flowing in the exhaust line through the turbocharger and supplying the portion of the low-temperature exhaust gas to the urea decomposing device as an additional gas for cooling a heating gas for urea decomposition; and a control portion configured to control overall SCR operations, including the control of fluxes in the first, second and third bypass lines (13, 14, 16) according to temperature of the SCR reactor and temperature of the heating gas of the urea decomposing device. a SCR reactor; an exhaust line configured to guide a low-temperature exhaust gas passing through a turbocharger to the SCR reactor; a urea decomposing device having a urea decomposition chamber and configured to decompose urea to generate ammonia and send the generated ammonia to the SCR reactor; a first bypass line configured to extract a portion of a high-temperature exhaust gas before flowing from an engine into the turbocharger and guide the portion of the high-temperature exhaust gas to the urea decomposing device; a second bypass line configured to connect the first bypass line and the exhaust line to distribute the high-temperature exhaust gas flowing in the first bypass line to the exhaust line so that a flux of the bypass gas supplied to the urea decomposing device through the first bypass line is changed; a third bypass line configured to connect the exhaust line and the first bypass line to have a diverging point in the exhaust line at an upstream of a joining point of the second bypass line and have a joining point in the first bypass line at a downstream of a diverging point of the second bypass line, the third bypass line extracting a portion of the low-temperature exhaust gas flowing in the exhaust line through the turbocharger and supplying the portion of the low-temperature exhaust gas to the urea decomposing device as an additional gas for cooling a heating gas for urea decomposition; and a control portion configured to control overall SCR operations, including the control of fluxes in the first, second and third bypass lines (13, 14, 16) according to temperature of the SCR reactor and temperature of the heating gas of the urea decomposing device.