ZnO/GaN heterojunction is an ideal material for the next generation el ectronic devices because of its low lattice mismatch,excellent chemical,optica l,piezoelectric and thermoelectric properties.However,the wide band gap of Zn O/GaN heterostructure restricts its application.In order to reduce the band ga p of ZnO/GaN heterojunction and improve the absorption of visible light,the eff ect of Cu doping on the stability,band gap and optical properties of ZnO/GaN he terojunction was studied by first principles.The results show that the binding energy of Cu-doped ZnO/GaN heterojunction is negative,so the structure is stab l e.However,the binding energy of Cu at the interface is smaller than that in th e matrix,so it is easier to form at the interface.Ga substitution with Cu can obviously reduce the band gap,and Cu-3d impurity state is introduced into the b and gap which is beneficial to the absorption of visible light.However,it is n ot obvious after Zn substitution.Red shift phenomenon appears in absorption coe fficient of Cu doped ZnO/GaN,and Ga substituted with Cu has an obvious absorpti on peak of 55377cm－1 near 3.2eV.Cu doping also cause s red shift of imaginary pa rts of dielectric function and refractive index,which indicates that doping is beneficial to the absorption of visible light.Although the reflectivity of Ga s ubstitution with Cu has been greatly improved,but the overall level is still lo w,so the reflectivity is low.