The mass transfer performance and influence factors on dissolved oxygen inside of ammonium oxidizing bacteria biofilm in fluid-bed bioreactor were analyzed based on the theatrical modelling and experiments. One-dimensional modeling approach for this study based on classic model of reac-tion-diffusion-convection mixing was developed and solved. It is released that the seepage velocity and diffusivity have a great effect on the max depth DO can get to; the higher the seepage velocity and diffu-sivity and feeding matrix concentration are, the deeper in biofilm dissolved oxygen can reach and the higher DO concentration is inside of the biofilm. A sequencing batch biofilm reactor (SBR) was set up to culture aerobatic biofilm enriched by ammonia oxidation bacteria (AOB) on K1 carrier. Organic was add-ed to accelerate the biofilm culture process in the startup phase. More than 80% of ammonia in the in-fluent was converted to nitrite and the biofilm with a thickness of 0.1-0.3 mm was achieved. Oxygen concentration along the direction of the biofilm depth was tested by PA2000 oxygen microelectrode. The results from both theatrical modelling and experiments showed the same DO gradient rules in the biofilm and experimental results agreed well with the model.