The invention provides an LED epitaxial structure and a preparation method thereof. An epitaxial wafer comprises a substrate, a first semiconductor layer, a multi-quantum well layer and a second semiconductor layer, the multi-quantum well layer comprises a quantum well layer and a composite quantum barrier layer; the composite quantum barrier layer comprises a Be-doped Al < z > Ga < 1-z > N layer, and the doping concentration of Be and the Al component in the Al < z > Ga < 1-z > N layer are gradually increased in the direction from the N-type GaN layer to the P-type GaN layer, so that the problems that in the prior art, effective electron hole recombination mostly occurs in the last few quantum wells of the multiple quantum wells, the effective radiation recombination light-emitting area is small, the light-emitting efficiency is influenced, and the light-emitting efficiency is influenced are solved. Meanwhile, Mg doping ionization energy is high, the concentration of holes in P-type GaN is limited, and then radiation recombination efficiency is affected.