The co-gasification of biomass and polyethylene in an updraft gasifier can effectively inerease the calorific value of the syngas and the H_2 yield, However , the generation and emission characteristics of nitrogen-containing pollutants during the co-gasification process have not yet been fully understood. Therefore, this study constructed a two-dimensional mathematical model of gas-solid flow coupling with chemical reaction in updraft gasifier based on the particle dynamies theory and Euler-Euler method, and investigated the effects of air equivalence ratio (ER), the blending ratio of biomass and polyethylene, and steam-to-fuel ratio (S/F)on the generation characteristics of nitrogen-containing pollutants. The results showed that NH_3 and HCN mainly came from the pyrolysis of biomass volatiles during the co-gasification process of biomass and polyethylene, and therefore they were mainly distributed in the pyrolysis zone and dry zone of the updraft gasifier. The generation of NO mainly came from the combustion and gasification reaction of char, and thus the oxidation zone had the highest NO content. NO then reacted with char, CO and H_2, leading to the decrease of its content. With the increase of air equivalent ratio, the content of NO gradually increased from 5.17 ppmv to 50.10 ppmv, while the content of other nitrogen pollutants such as N_2O, NH_3 and HCN decreased. Increasing the moisture vapor to fuel ratio reduced NO content from 33.74 ppmv to 0.95 ppmv, but promoted the generation of N_2O. The introduction of polyethylene could effectively reduce the generation of nitrgen-containing pollutants during biomass gasilication, and the synergistic effect between biomass and polyethylene during co-gasification process could further inhibit the generation of nitrogen-containing pollutants such as NO, NH_3 and HCN.