The invention provides a preparation method of a red and yellow light emitting diode epitaxial wafer, and belongs to the technical field of diode preparation. The surface, far away from the substrate, of the epitaxial wafer, namely the surface, far away from the epitaxial layer, of the substrate is adhered to the supporting film, the supporting film can play a role in uniformly supporting the whole epitaxial wafer and can be stably adhered to the epitaxial wafer, and the surface, far away from the epitaxial layer, of the substrate can be uniformly thinned while operation is facilitated. An epitaxial wafer is placed on a supporting table through a supporting film, a substrate of the epitaxial wafer is thinned through a grinding wheel, the axis of the grinding wheel is perpendicular to the surface of the substrate, the projection area of the grinding wheel on the plane where the surface of the substrate is located is larger than the surface area of the substrate, it is guaranteed that the face, away from the epitaxial wafer, of the substrate is thinned stably through the grinding wheel, and the thickness of the substrate is evenly reduced; and the uniformity of the thickness of the finally obtained substrate is improved, so that the light emitting uniformity of the finally obtained red and yellow light emitting diode is ensured.