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In order to explore the performance of biochar-based microbial immobilization body in ammonium removal from water and potential mechanisms, a strain of heterotrophic nitrifying bacteria (HNB) was isolated from activated sludge, and the biochemical and molecular biological identification of HNB was carried out. Moreover, HNO₃-, Mg²⁺-, NaOH_-, and NaOH+Mg²⁺-modified rice husk-derived biochars were prepared. Then all the five kinds of biochars, including the original biochar, were used as carriers of HNB to remove NH₄⁺_-N from water. Results showed that HNB was classified as <i>Pseudomonas</i>, and the 72-h NH₄⁺_-N removal ratio of the free bacteria reached 80.24%. Compared with biochar itself, biochar-based HNB immobilization body showed a much stronger ability to remove NH₄⁺_-N, especially for NaOH_- and NaOH+Mg²⁺-modified biochars. At the initial NH₄⁺_-N concentration of 100 mg/L and biochar addition dose of 10 g/L, NH₄⁺_-N removal ratio of NaOH_- and NaOH+Mg²⁺-modified biochar-based HNB immobilization bodies reached 57.78% and 58.35% after 5 h, and reached 88.66% and 90.93% after 48 h respectively, which were obviously higher than the original, HNO_3- and Mg²⁺-modified biochar-based HNB immobilization bodies. The phenomenon resulted from significantly higher bacteria adsorption ability of NaOH_- and NaOH+Mg²⁺-modified biochars, which reached 773.75 and 941.17 nmol P/g biochar, respectively.