Polyamidoamine (PAMAM) dendrimers with graphene as core encapsulated Co nanocomposites which was prepared by chemical reduction method. The samples were characterized by means of FTIR, TG, XRD and TEM. The effect of pH value, adsorption time and temperature conditions on the third generations of PAMAM dendrimer with graphene as core nanocomposite (G3.0-PAMAM/Co) tested for Congo red adsorption performance were studied. The results showed that G3.0-PAMAM/Co was strong magnetic adsorbent for Congo red. The adsorption process of Congo red on G3.0-PAMAM/Co could be described with the Lagrangian pseudo-second-order kinetic model. The results indicated that the adsorption process was mainly chemical adsorption. The thermodynamics of Congo red adsorption on the G3.0-PAMAM/Co followed the Langmuir adsorption equation. The maximum adsorption amount was 161.49 mg/g.