Abstract: Nitrogen oxides (NO_x), the precursors of PM_2.5 and O₃, are important air pollution control indices. Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are mature technologies for NO_x removal from coal-fired industrial boiler flue gas. However, the application of the two technologies in small-and-medium denitrification engineering is limited because of their high investment costs, harsh operating conditions and other factors. In recent years, biological denitrification technologies have been used increasingly in small-and-medium flue-gas denitration projects. Many scholars have carried out extensive research on them. The advances of bioprocesses for NO_x removal from flue gas were critically reviewed, and the denitrification principles and technical characteristics of related processes were summarized. The latest research directions of chemical absorption-biological denitrification (BioDeNO_x) were reviewed, and the operational principles, reduction mechanism, reactor developments, operational parameters and influencing factors of complexation absorption-biological reduction (CABR) were emphatically elucidated. The obstacles of the CABR system and its solutions were systematically discussed, and the future trends of bioprocesses for NO_x removal were prospected.