The invention discloses an LED epitaxial wafer and a preparation method therefor, and belongs to the technical field of semiconductors. The epitaxial wafer includes a substrate, a buffer layer, an N-type semiconductor layer, a multiple-quantum-well layer and a P-type semiconductor layer, wherein the multiple-quantum-well layer includes n quantum wells and (n+1) quantum barriers, which are alternately stacked. Each quantum barrier includes a first sub-layer, a second sub-layer, a third sub-layer, a fourth sub-layer and a fifth sub-layer, wherein the first, second, third, fourth and fifth sub-layers are stacked together, and the first and fifth sub-layers are an N-type doped gallium nitride layers, the second and fourth sub-layers are undoped gallium nitride layers, the third sub-layer is anundoped aluminum nitride layer. The doping concentration of an N-type dopant in the first sub-layers of (n+1) quantum barriers decreases layer by layer in the lamination direction of the epitaxial wafer, and the doping concentration of the N-type dopant in the fifth sub-layers of (n+1) quantum barriers decreases layer by layer in the lamination direction of the epitaxial wafer. The luminous efficiency of an LED can be improved.