The invention discloses an epitaxial wafer for a gallium-nitride-based light emitting diode, and a preparation method for an epitaxial wafer, and belongs to the technical field of semiconductors. The epitaxial wafer comprises a sapphire substrate, a GaN buffering layer, a non-doped GaN layer, an N-type GaN layer, a multi-quantum-well layer, a P-type electron blocking layer and a P-type layer, wherein the GaN buffering layer, the non-doped GaN layer, the N-type GaN layer, the multi-quantum-well layer, the P-type electron blocking layer and the P-type layer are sequentially stacked on the sapphire substrate. The P-type layer is formed by the alternate stacking of a P-type GaN layer and a graphene film layer. According to the invention, the P-type GaN layer and the graphene film layer are alternately stacked to form the P-type layer, and the graphene film layer can prompt the hole transmission and also can improve the lateral extension capability of hole. Moreover, the graphene film layer also can improve the energy band matching of the P-type electron blocking layer and the P-type GaN layer, improves the electrical contact of the P-type electron blocking layer with the P-type GaN layer, improves the capability of a hole injection multi-quantum-well layer and finally improves the luminous efficiency of a light-emitting diode.