The micro-mechanism of heavy metal cations adsorption onto biochar is critical for the renovation of heavy metal contamination. In this work, we prepared three mesoporous cellulose biochar (MCB) adsorbents with different surface area, O and N functionalities content properties through a 300 degrees C carbonization-KOH activation-700 degrees C carbonization-HNO3 oxidation process. The Cd(II) adsorption performance of three MCB was compared, the MCB-1h exhibited best Cd(II) adsorption capacity (368.8 mg/g), and even remained 89% after 5 cycles. DFT calculations identified that MCB-1h had comparatively stronger Cd(II) binding ability, and the results were well consistent with the experimental data. The adsorption micro-mechanism was analyzed in the view of classical theory and electron-scale. The classical theory results proposed that the adsorption process was dominated by chemisorption, and electron sharing or exchange between Cd(II) and biochar occurred. Electron-scale mechanism analysis found that the functional groups and aromatic could provide lone pair electrons and pi electrons for the Cd(II) adsorption, respectively. The N functionalities such as amino, pyridine and pyrrole groups could raise the adsorption ability of the biochar adsorbent. Overall, our results not only provide new insights into the heavy metal adsorption, but also has significant reference value for the subsequent biochar adsorbent preparation.