The invention relates to a method for preparing a SiCN-hafnate yttrium complex coating. SiCN ceramic powder, nanometer yttrium oxide powder and nanometer hafnium oxide powder are mixed evenly and then dispersed in acetylacetone to obtain suspension, elemental selenium and graphene subjected to analytically pure concentrated nitric acid activating treatment are added, and finally the dense and uniform C/SiC composite material SiCN-hafnate yttrium complex coating is prepared through pulse electrophoretic deposition and spark plasma sintering. Compared with the prior art, the method has the advantages that the outer coating is dense and high in crystallinity, strength, tenacity and interface bonding performance, the porosity of the coating is effectively reduced, and the high-temperature CMAS corrosion resistance of the coating is improved greatly.