The invention relates to a method for preparing a nylon additive N,N'-bis(2,2,6,6-tetramethyl-4-piperidinyl)-1,3-phenyldicarbonamide. According to the method, m-phthalic dialkyl ester and tetramethylpiperylhydrazine, which are used as raw materials, react in a nonpolar organic solvent under the pressurized condition to generate a target product. In the method, no catalyst is used, and less wastewater is generated. The method is simple to operate and is suitable for industrial production. Meanwhile, GC content of the target product obtained by the method can reach 99.8% and above, and yield canreach 95% and above.