the present application discloses a composite substrate with a protective layer for preventing metal from diffusing , comprising : a thermally and electrically conductive layer having a melting point of greater than 1000 ° c ., and a gan mono - crystalline layer located on the thermally and electrically conductive layer . at least the side wall of the composite substrate is cladded with a protective layer for preventing metal from diffusing . the composite substrate not only takes account of the homoepitaxy required for gan epitaxy and improves the quality of the crystals , but also can be used directly to prepare leds with vertical structures and significantly reduce costs . the disclosed composite substrate effectively avoids the pollution of experimental instruments by the diffusion and volatilization of a metal material during the growth of mocvd at high temperature .