The invention provides an LED wafer and a preparation method thereof and an LED lamp. The preparation method comprises the following steps: 1) growing an AlN composite substrate on a sapphire substrate; 2) growing a layer of Al0.5Ga0.5N quantum structure active layer on the AlN composite substrate; 3) epitaxially growing a layer of thin AlN buffer layer on the Al0.5Ga0.5N quantum structure activelayer to serve as a hole blocking layer; 4) growing multiple layers of quantum well structures on the AlN buffer layer; 5) growing a P-type electron barrier layer on the quantum well structures; 6) growing Mg-Si[delta] co-doped superlattices on the P-type electron barrier layer; and 7) growing a layer of P<+>-GaN ohmic electrode cover layer on the Mg-Si[delta] co-doped superlattices. The beneficial effects are that, according to the preparation method of the LED wafer comprising a high-Al-component AlGaN epitaxial substrate, by growing the AlN composite substrate on the sapphire substrate, stress in the growth process is released effectively, and the high-quality AlGaN epitaxial substrate can be obtained.