The nitrogen loading rate (NLR) of an anammox attached film expanded bed (AAFEB) reactor was increased from 5.0 to 60.0 gN/L/d. During the stable operational period, the TN removal efficiency maintained at 87.3 +/- 2.5%, and a maximum nitrogen removal rate (NRR) of 44.9 +/- 0.3 gN/L/d was achieved. Overload resulted in the sharp deterioration of reactor performance, the ratio of (Food/Microorganism)/SAA should be maintained at lower than 66 +/- 7% to ensure the stable operation of the AAFEB reactor. New stoichiometric equations for the anammox process under the low NLR condition (5.0 gN/L/d) and the high NLR condition (50.0 gN/L/d) were proposed. The quantitative SAA-cytochrome heme C relationship was established for the first time that providing a simple way for monitoring the reactor performance. Substrate tolerance ability was significantly increased that proving the stability of the AAFEB reactor was continuously enhanced during the stable operational periods.