Abstract: A biofilter packed with activated carbon was set up to study the removal of nitrogen oxides of low concentration under high flow load. The experimental result showed that the best NOx purifying efficiency reached 99%. The optimum sprinkling amount and gas flow were 3L/h and 0.4—0.5m3/h，respectively. When gas flow was 0.56m3/h，NOx concentration in inlet gas was 480 mg/m3 and residence time was 18.4s，the purification efficiency of NOx could reach 96.67%. The main denitrifying bacteria were Paracoccus and Thiobacillus; the biological purification reaction took place mainly not on internal surface but on external surface of activated carbon attached with biofilm .