A one-dimensional biofilm model was applied to illustrate the nitrogen conversion and removal within the heterogeneous biofilm attached on the gas-permeable membrane with different oxygen transfer coefficients: 7.5m/d, 1.5m/d and 0.3m/d. Integrating the ammonia-oxidizing bacteria-mediating hydroxylamine oxidization pathway during the autotrophic nitrification and the four-step denitrification pathway during the heterotrophic denitrification, the effects of the intra-membrane aeration pressure and the influent COD/N ratio were further quantitatively evaluated on the systematic performance of nitrogen conversion. Dynamic profiles of key nitrogenous intermediates were investigated to further analyze the treatment efficacy of the targeted biofilm system. It is inapplicable for membrane with oxygen transfer coefficient of 0.3m/d to sustain the biofilm due to the inferior treatment performance under higher influent organics and distinct nitrous oxide (N2O) production with elevated aeration pressures under lower influent organics. For the oxygen transfer coefficients of 7.5m/d and 1.5m/d, N2O production was detectable for the insufficient carbon source, indicating the significance of hydroxylamine oxidization. Short-cut nitrogen removal pathway could be feasible within the latter biofilm due to the nitrite accumulation, further reduced by supplementing the carbon source. Heterotrophic denitrification would contribute to the N2O production. Maintaining the biofilm thickness was conducive to short-cut nitrogen removal by regulating the substrate transfer and the biomass distribution along the biofilm. Besides the total nitrogen removal efficiency, the nitrite accumulation and N2O production were both decreased with the thickening biofilm. Inside the thinner biofilm, a short-cut pathway via nitrite might be the major pathway for nitrogen removal with distinguished N2O production, which could be mitigated through supplementing the carbon source.