The invention provides a self-supporting gallium nitride layer and a preparation method thereof. The preparation method at least comprises the following steps: providing a substrate; forming a first gallium-containing decomposed layer, a decomposed blocking layer, a second gallium-containing decomposed layer and a graphical mask layer on the substrate in sequence, wherein a plurality of openings are formed in the graphical mask layer; carrying out thermal annealing, so that the first gallium-containing decomposed layer and the second gallium-containing decomposed layer are partially decomposed and reconstructed, and a first decomposed and reconstructed layer internally provided with a plurality of first holes, a second decomposed and reconstructed layer internally provided with a plurality of first holes and gallium nitride crystal seed layers positioned inside the openings are formed respectively; carrying out thermal growth, so that the gallium nitride crystal seed layers grow, and a gallium nitride layer is formed on the graphical mask layer; and carrying out cooling, so that the gallium nitride layer is automatically peeled off from the substrate so as to obtain the self-supporting gallium nitride layer. The preparation method is relatively low in preparation process requirement, and quick self-peeling for the gallium nitride layer can be realized, so that the self-supporting gallium nitride layer with high finished product rate can be obtained.