In this experiment, a rapid mass-transfer inner loop fluidized bed biofilm reactor (ILFBBR) was employed to treat synthetic high ammonia nitrogen-containing (NH4+-N) wastewater by shortcut nitriﬁcation–denitriﬁcation. The reactor operation was stable after a short start-up period. Ammonia oxidizing bacteria (AOB) were predominant and 65% nitrite (NO2−-N/NOx−-N) levels were achieved. During the nitriﬁcation–denitriﬁcation period, the removal rates of NH4+-N and total nitrogen (TN) reached 94 and 82%, respectively. From the material balance, it was indicated that 87% of NH4+-N was removed by shortcut nitrification. The features of ILFBBR and the beneﬁts of shortcut nitrification were combined in this experiment, and showed an excellent removal of NH4+-N from high-concentration NH4+-N wastewater.