A novel biosorbent (MSBP) was synthesized in this paper by microwave-assisted esterification with maleic anhydride (MA) using sugar beet pulp (SBP) as raw material. Characterization of prepared MSBP was done by measuring TG/DTG, BET surface area analysis, XRD and FT-IR analysis. Then MSBP was used for the adsorption of methylene blue (MB), and the experimental parameters such as microwave time, microwave power and mixing ratio were optimized. The water retention capacity and antibacterial activity of MSBP were determined. The results showed that the optimum modification conditions were a microwave time of 2 min, a microwave power of 1000 W, and a mixing ratio of 15:3 between MA and SBP, under which the cation adsorption capacity of MSBP was 343.35 mg/g. In addition, the esterification reaction allowed MSBP to form a tightly cross-linked structure, resulting in lower water retention capacity. At the same time, the modified MSBP also showed antibacterial activity against E. coli and S. aureus. In summary, a simple and efficient monofunctional modification method was used in this study, and a green sugar beet pulp-based biosorbent was successfully prepared.