Activated carbon (AC) was modified by MgO and MnO through an impregnation-precipitation-calcination procedure. The batch experiments of adsorption of Rhodamine B (RB) by a modified adsorption material, an MgO-MnO-AC composite, were carried out and the characteristics of the composite adsorbent were evaluated. The results showed that manganese/magnesium loading changed the surface area, pore volume and increased the number of active adsorption sites of AC. The highest Brunauer-Emmett-Teller (BET) surface area (1,036.18 m·g) was obtained for MgO-MnO-AC compared with AC. The content of AC loaded with magnesium and manganese was 34.24 and 5.51 mg·g respectively. The adsorption of RB on MgO-MnO-AC was significantly improved. The maximum adsorption capacity of RB on MgO-MnO-AC was 16.19 mg·g at 25 °C under the RB concentration of 50 mg·L. The adsorption of RB by AC and MgO-MnO-AC increased with the initial concentration of RB. The adsorption of RB increased first and then decreased when pH was between 3 and 11. The results indicated that the pseudo-second-order kinetic equation and Langmuir equation can be used to describe the adsorption of RB on MgO-MnO-AC.