To provide an exhaust emission control system of a construction machine which has prolonged life of a DPF (Diesel Particulate Filter) by suppressing the generation of PM (Particulate Matter) thereby reducing the frequency of regenerative combustion.

The exhaust emission control system of the construction machine includes: an engine; a motor which is power-coupled with the engine and which can be controlled into a state in which the motor generates power by the power generated by the engine and a state in which the motor performs torque assist to the engine and a driving shaft; a hydraulic pump connected to the driving shaft; and the PDF for collecting the PM contained in the exhaust gas. The exhaust emission control system further includes: a pressure sensor detecting the discharge pressure of the hydraulic pump; a smoke meter detecting smoke concentration of the engine: and a controller which performs a first control mode for setting correlation characteristics and a threshold based on the discharge pressure detected by the pressure sensor and the smoke concentration detected by the smoke meter and a second control mode keeping the smoke concentration equal to or below the threshold value by controlling the amount of the power assist of the motor when the detected smoke concentration exceeds the threshold value.