In order to improve the adsorption performance of activated carbon, magnesium oxide modified activated carbon (MgO/GAC) was prepared by equal volume impregnation with magnesium nitrate and activated carbon, adopting high temperature roasting. The influence of temperature and adsorption time on the adsorption of low-concentration reactive red dye in wastewater was tested by the composite material. The results showed that the iodine adsorption value of MgO-GAC was 960.42mg/g with 3.5mol/L magnesium nitrate impregnation, calcination temperature 600℃, calcination time 2h. Scanning electron microscopy (SEM) photos showed that the surface of the unmodified granular activated carbon had the micropore diameter of about 3μm, and the surface of the modified MgO-GAC composite had hte uniform micropore size, with the pore size of about 6-7μm. The surface was loaded with a large number of small round particles. High-temperature roasting had a pore expansion effect on granular activated carbon, and could convert magnesium nitrate into porous magnesium oxide, which could be effectively loaded on the surface of granular activated carbon. The optimal conditions for the adsorption of Reactive Red X-3B dye with the MgO-GAC composite material were as follows: the dosage was 0.1 g, the temperature was 30℃, the pH value was 6, and the removal rate of reactive red dye reached 92.5%. The preparation method of modified activated carbon was feasible,high temperature expansion and porous magnesia increased the surface area of granular activated carbon,which improved the adsorption effect of the reactive red dye.