A process for treating nitrogenous wastewater contains an autotrophic denitrification reaction, a heterotrophic denitrification reaction and a COD removal reaction simultaneously and mixedly taking place in a single reactor. The nitrification reaction is caused by nitrifying bacteria, in which ammonium is oxidized into nitrite. The autotrophic denitrification reaction is caused by autotrophic denitrifying bacteria, in which ammonium used as electron donor and nitrite used as electron acceptor are converted into nitrogen gas and nitrate. The heterotrophic denitrification reaction is caused by heterotrophic denitrifying bacteria, in which nitrate and COD are consumed. It is not necessary to build two separate reactors for aerobic nitrification and anaerobic denitrification, thereby effectively reducing the fabrication and operation cost.