Nitrogen-doped bagasse biochar BAC-EDA was prepared via a two-step process, namely, the bagasse biochar was prepared with bagasse as raw material and phosphoric acid as modifying agent and then on which nitrogen-containing functional groups were further coupled with ethylenediamine as dressing agent. The pore structure of BAC and BAC-EDA was characterized by means of BET, BJH and t-plot methods based on the experimental data of liquid nitrogen adsorption/desorption. Meanwhile acidic groups were characterized by FTIR method. Results show that pores of biochar BAC and BAC-EDA are mainly of mesopores with a small amount of micropores. There existed oxygen-containing acidic functional groups on the surface the nitrogen-doped bagasse biochar BAC-EDA, such as carboxyl groups, lactone groups and amino nitrogen-containing functional groups etc. The proportions of the micro-mesopores of the biomass carbon are constant before and after modification, which indicating that the introduction of the functional groups is realized simultaneously on the surface of both the micro- and meso-pores. Adsorption experiments show that the maximum adsorption amount of the modified BAC-EDA is 137 mg·g^-1, much higher than that of the plain BAC 73 mg·g^-1, indicating that the nitric acidoxidation and ethylenediamine modification effectively improve the adsorption capacity of biomass biochar. The Langmuir model can better describe the adsorption of Hg(Ⅱ) by BAC-EDA, further illustrating the homogeneity of the active sites of the modified carbon. Besides, the temperature is conducive to adsorption, showing that the process is a spontaneous endothermic process.