In this study, activated carbons were prepred by the modified grapefruit peel with different methods. The effects of the pH value, the dosage of activated carbon, the initial concentration of methylene blue and the adsorption time on the adsorption effect were investigated. The adsorption kinetics and adsorption mechanism were also discussed. The results showed that under the optimal conditions, the removal rates of methylene blue by four activated carbons were 84.5% (unmodified), 87.3% (modified by aluminum chloride impregnation method), 91.1% (modified by sodium sulfide activation), and 95.5% (modified by potassium hydroxide activation), respectively. Comparing the value of the equilibrium adsorption capacity of different activated carbons, the activated carbons prepared from modified pomelo peel with aluminum chloride and potassium hydroxide had a significantly improved adsorption ability. The analysis of adsorption kinetics showed that the adsorption process of methylene blue on four kinds of activated carbons all accorded with the quasi-second-order kinetic model. The surface of activated carbons prepared from modified pomelo peel with aluminum chloride and potassium hydroxide contained many organic functional groups, which was consistent with the higher initial concentration of methylene blue wastewater and high equilibrium adsorption of activated carbons.