Aiming at the pollution problem of methylene blue dye wastewater, a new type of methylene blue adsorbent magnesium-modified fly ash (Mg@FA) was prepared by using solid waste fly ash as raw material. The effects of Mg@FA dosage, adsorption time, and methylene blue concentration on the adsorption of methylene blue by Mg@FA and pH values were analyzed. The adsorption characteristics of Mg@FA on methylene blue were investigated by adsorption kinetics, adsorption isotherms, and adsorption thermodynamics, as well as SEM, EDS, XRD, BET, and FTIR. The results showed that when the dosage of Mg@FA was 1.0 g, the adsorption time was 120 min, and the initial concentration of methylene blue was 150 mg/L
the adsorption efficiency of methylene blue by Mg@FA was the highest, which was 95.61%. When the pH of the methylene blue solution was in the range of 7-11, the adsorption efficiency of Mg@FA for methylene blue remained stable at 95.61-98.10%. The adsorption process of methylene blue by Mg@FA follows the second-order kinetic fitting model and Langmuir model. The adsorption of methylene blue by Mg@FA is a spontaneous and endothermic reaction. Mg@FA adsorbs methylene blue through electrostatic interaction and hydrogen bonding. Mg@FA can effectively adsorb methylene blue and promote the waste utilization of fly ash, which provides a promising method for wastewater treatment and fly ash utilization.