The invention discloses a boiler flue and a denitrification system. The boiler flue comprises a vertical flue part and a turning flue part, wherein the vertical flue part is connected with the turningflue part, an ash damper and a straight plate are obliquely installed on the inner side wall of the vertical flue part, and an arc-shaped plate is installed on the turning flue part. According to theboiler flue, the ash damper and the straight plate are obliquely installed on the vertical flue part, so that the speed and distribution of smoke in the vertical flue part are adjusted; the arc-shaped plate is arranged on the turning flue part, so that the speed and distribution of smoke in the turning flue part are adjusted, the flow speed and distribution of the smoke in the whole boiler flue are homogenized, and the technical problem of uneven flow distribution in the boiler flue of an existing denitrification system is solved.