Abstract: Tail water containing relatively high concentration of ${\mathrm{NO}}_3^{-}$ -N is often discharged even after secondary treatment process. A biomass-sulfur mixed denitrification system was constructed for the treatment of elevated concentration of ${\mathrm{NO}}_3^{-}$ -N in tail water, and its performance for the removal of ${\mathrm{NO}}_3^{-}$ -N was studied. The results of static experiments showed that woodchips-sulfur mixed denitrification system had good performance for the removal of nitrate (reaction rate constant 0.041 6 d^-1) with the least conversion of ${\mathrm{NO}}_3^{-}$ -N and ${\mathrm{NH}}_4^{+}$ -N. The initial pH within the range of 6-9, and the woodchip/sulfur ratio of 0.5-2.0 had insignificant effect on the mixed denitrification process. The addition of iron fillings had a certain regulating effect on the pH of the system. In the dynamic experiments, mixed denitrification system with 50 g sulfur plus 25 g woodchips had a good removal effect on ${\mathrm{NO}}_3^{-}$ -N with initial concentration of 15 mg/L , and the removal efficiency of ${\mathrm{NO}}_3^{-}$ -N could reach 90%. The removal rate of high ${\mathrm{NO}}_3^{-}$ -N concentration of mixed denitrification system could be improved by changing the proportion of woodchips and sulfur or adding iron sawdust. Among the trials, mixed denitrification system of 50 g sulfur and 50 g woodchips could maintain a removal efficiency of 90% even for 30 mg/L of ${\mathrm{NO}}_3^{-}$ -N. Different initial ${\mathrm{NO}}_3^{-}$ -N concentrations had some effect on the abundance of microbial community in the reaction column, but had little effect on the structure of microbial community.