Abstract: In order to achieve rapid enrichment of anaerobic ammonia-oxidizing bacteria (ANAMOB) in the carrier biofilm, nano-iron modified biochar carrier (nZVI@BC) was prepared by liquid-phase reduction, and the surface properties of biochar (BC) and nZVI@BC were compared by the characterization methods, such as specific surface area, scanning electron microscopy and X-ray diffraction. Two sets of identical anaerobic ammonia oxidation (ANAMMOX) enrichment devices were set up, and R₁ and R₂ were added with BC carrier and nZVI@BC carrier, respectively. The effects of different carriers on the nitrogen removal performance of the ANAMMOX device in the start-up phase and the concentration increase phase were analyzed; the color change, the amount of microorganisms in the surface biofilm and the surface morphology of the biofilm of different carriers were compared to investigate the enrichment effect of the different carriers on AnAOB; and the differences in the structure of the microbial community in the biofilm of the surface biofilm of different carriers were investigated by using the high-throughput sequencing technique of 16s rRNA. The results showed that: (i) nZVI was successfully loaded onto BC using the liquid phase reduction method. nZVI@BC carrier has higher specific surface area and porosity than BC carrier, which can provide more attachment sites for microorganisms. (ii) After 40 d of operation, the ANAMMOX device was successfully started, and the total nitrogen removal loads of R₁ and R₂ reached 0.25 and 0.2554 kg·N/(m3·d), respectively. (iii) The color of the carrier surface changed from black to reddish brown, and the red ANAMMOX biofilm was initially formed. And the surface biomass of nZVI@BC carrier was 23 times higher than that of BC carrier. (iv) At the 20th d, the relative abundance of AnAOB in R₁ and R₂ was 10.47% and 29.15%, respectively; the main genus of AnAOB on the surface of R₁ and R₂ carriers was Candidatus_Kuenenia, and its relative abundance was 7.49% and 23.76%, respectively. Therefore, the modified nZVI@BC carriers are more helpful for the rapid formation of ANAMMOX biofilm and enhance the denitrification performance of ANAMMOX.