The invention discloses an AlGaN-based deep-ultraviolet LED epitaxial structure and a preparation method thereof, which utilize Mg-doped quantum wells to enhance luminous efficiency. The deep ultraviolet LED structure includes a substrate, a buffer layer, an AlN layer, a superlattice stress regulation / dislocation filter layer, a non-doped AlGaN layer, an n-type AlGaN layer, a Mg-doped active light-emitting region multi-quantum well layer, A p-type AlGaN layer and a p-type GaN contact layer. In the invention, Mg impurity is doped in the middle third of the well layer of the multi-quantum well active luminescent layer of the LED, so as to improve the internal quantum efficiency and light extraction efficiency of the LED. Compared with the non-doped multiple quantum well structure, the Mg-doped multiple quantum well structure can suppress the quantum confinement Stark effect, improve the spatial overlap of electron and hole wave functions and radiative recombination efficiency, and provide more holes Participate in radiative recombination and improve internal quantum efficiency. Moreover, Mg doping can also introduce a local strain field, increase the compressive strain in the quantum well, increase the ratio of TE polarized light, and finally improve the light extraction efficiency of AlGaN-based deep ultraviolet LEDs.