Abstract: As the performances of traditional biomass materials are poor in absorbing ammonia nitrogen in wastewater, they are often engineered to produce biochar or activated carbon as adsorbent to improve the adsorption performance. Camellia oleifera shell (COS) is a natural biomass enriched with tannic acid, which can be used to remove pollutants in water as an appropriate adsorption material after surface modification. In this study, complex biomass material (Fe/COS) was prepared based on Camellia oleifera shell. The ammonia nitrogen adsorption characteristics of Fe/COS were investigated through adsorption batch experiments, the composition and structure of Fe/COS were analyzed by SEM, XRD, FT-IR, and BET, and then the adsorption mechanism was discussed. The results show that the material has excellent ammonia nitrogen removal effect in water at pH 7~9, and the adsorption of ammonia nitrogen fits well with the pseudo-second-order kinetic model (R²>0.99), and the chemical adsorption is the dominant process. The adsorption isotherm follows the Langmuir model (R²>0.99), illustrating that it is assigned to monolayer adsorption. The saturation adsorption capacity is 19.18 mg·g⁻¹ at 290 K. Moreover, the common co-existing ions in water have no obvious effect on the adsorption of ammonia nitrogen at higher concentration, indicating that Fe/COS has good adsorption selectivity for ammonia nitrogen. The adsorption mechanism of ammonia nitrogen by Fe/COS could be mainly attributed to coordination, electrostatic attraction, and ion exchange. In summary, biomass-based adsorbents prepared by the complexation of Fe³⁺ with tannic acid on the surface of Camellia oleifera shell demonstrate exceptional adsorption performance. This work provides a theoretical basis for the application of Camellia oleifera shell to remove ammonia nitrogen in the wastewater.