The present invention discloses a synthetic process for an intermediate of a plant protection agent, which relates to the technical field of synthesis of nitrogen-containing heterocyclic compounds. The synthetic process comprises the following steps: dehydration condensation reaction of tert-butylurea and ethyl acetoacetate in the presence of a substituted aromatic hydrocarbon solvent and a phosphorus-containing catalyst to generate 3-(3-tert-butyl)urea-2-butenoate; and further cyclization reaction under alkaline conditions to obtain 3-tert-butyl-6-methyluracil. The present invention optimizes the catalyst and solvent for the dehydration condensation reaction, and significantly improves the yield and purity of 3-tert-butyl-6-methyluracil compared with the prior art, and has low production cost, stable and repeatable process, and meets the requirements of commercial production.