To realize the effective utilization of reed for adsorption of methylene blue (MB), modified reed activated carbon (M-RAC) was prepared using reed as raw material and MgCl2 and FeCl3 as activators. The four parameters of M-RAC dosage, MB concentration, adsorption time, and oscillation velocity during the adsorption process were optimized by the response surface methodology (RSM). The adsorption performance of M-RAC on MB under the optimal process was determined and analyzed by three different adsorption models. The results showed that the adsorption capacity of M-RAC on MB was 436.33 mg/g when the M-RAC dosage was 0.17 g, the MB concentration was 550 mg/L, the adsorption time was 105 min, and the oscillation velocity was 154 r/min. The experimental data were consistent with the Langmuir model and Pseudo-secondorder kinetics, which indicated that the adsorption process was a monolayer chemical adsorption process. The thermodynamic study confirmed that the increase in temperature favored the adsorption of M-RAC on MB, and the adsorption process was a spontaneous and heat-absorbing reaction. The analysis of the Fourier transform infrared (FTIR) spectroscopy showed that the absorption peaks of hydroxyl functional groups in the prepared M-RAC were higher, and the C-O characteristic peaks were enhanced. The results suggest that M-RAC is a potential biochar adsorbent for effectively removing MB dye from wastewater.