The invention relates to an epoxy resin curing agent and a preparation method thereof. The epoxy resin curing agent comprises the following raw materials in parts by weight: 30-60 parts of methacrylic acid dimethylamine ethyl ester, 20-30 parts of methyl methacrylate, 10-20 parts of isoamyl methacrylate, 10-20 parts of hydroxyethyl methacrylate, 0.5-3 parts of an initiating agent, and 50-100 parts of solvent. According to the invention, the methacrylic acid dimethylamine ethyl ester with unsaturated bond is adopted as a main comonomer, and tertiary amine which can be used for performing cross-linking and solidification reaction with the epoxy resin is directly inserted into a macromolecular chain, so that the epoxy resin can be solidified, the toughening modification effect is achieved, and a curing agent and a flexibilizer are not required at one time; therefore the cost is saved and the technology is simplified.