The invention provides a preparation method of a modified graphene oxide, comprising the following steps of: (a) reacting a phosphorus oxychloride compound, cyanuric chloride or diisocyanate with glycidol to obtain an intermediate product; and (b) dispersing graphite oxide in an organic solvent, dropwise adding to the intermediate product obtained through reaction, and reacting to obtain the modified graphene oxide. The invention also provides a preparation method of a composite material containing the modified graphene oxide, comprising the following steps of: dispersing the modified graphene oxide obtained through the preparation method in the technical scheme in an organic solvent, mixing the modified graphene oxide containing epoxide groups with epoxy resin oligomers and a polyamide curing agent, and curing to obtain the composite material containing the modified graphene oxide. In the composite material provided by the invention, the modified graphene oxide reacts with a resin substrate, is more uniform to disperse in the obtained composite material, and has better flame-retarding performance.