The invention provides a flue gas denitrification system for diesel engine exhaust afterburning. The system comprises a denitrification reactor, wherein a tail gas exhaust hole of a diesel engine is connected with the denitrification reactor through a tail gas flue and a bypass flue; a combustor and an ammonia agent ejection hole are arranged inside the bypass flue; and the ammonia agent ejectionhole is positioned on the rear side of the combustor. The invention further provides a flue gas denitrification system for diesel engine exhaust afterburning. Due to additional amounting of the bypassflue, diesel engine exhaust afterburning is implemented, the temperature of a flue gas can be increased, pyrolysis ammonia production can be improved, a pyrolysis furnace is not needed, the temperature of the mixed flue gas can meet requirements of selective catalytic reduction reactions, and meanwhile, the situation that the diesel engine is low in burden and too low in tail gas temperature canbe avoided.