• The unique structure of MgO/GNs can facilitate the transmission of electrons. • Graphene highly enhance the e−-h+ separation for its excellent electron migration. • Graphene with rich defects is beneficial to the visible light absorption. • MgO/GNs-2 performs the optimal photocatalytic activity (99.4% within 30 min). The 2D/2D MgO/graphene nanosheets (MgO/GNs-x) catalysts were successfully synthesized to improve the photocatalytic efficiency of MgO under visible light. The synergistic effect of MgO, GNs, and hydrogen peroxide (H 2 O 2) was investigated by degrading methylene blue (MB, 100 mg/L). The photocatalytic activity at 30 min of MgO and MgO/GNs-x (x = 1, 2, 3, 4, 5) under visible light were 89.0%, 99.0%, 99.4%, 98.0%, 97.8%, 84.0%, respectively. It can be found that the MgO with abound defects exhibited high photocatalytic activity. Further, the excellent electron migration of GNs was beneficial to separate photo-generated e−-h+ pairs and visible light absorption. The band gap E g is declined from 5.34 eV (MgO) to 4.94 eV (MgO/GNs-2). This study signifies that graphene effectively improves the photo-degradation efficiency of wide band-gap semiconductor photocatalyst. [ABSTRACT FROM AUTHOR]