The increasing resistance of weeds to long-term herbicide use presents a significant challenge in modern agriculture, necessitating the urgent development of new herbicides. Ageratina adenophora, a notorious perennial herb, has spread across pantropical and pansubtropical zones worldwide. In this study, we evaluated seven benzoic acid compounds as potential pesticides against A. adenophora and we identified methyl 4-hydroxyphenylacetate as an effective growth inhibitor for this weed. A 250 mM concentration of methyl 4-hydroxybenzoate dilution can completely kill A. adenophora plants that are 2 to 3 months old. For plants aged 5 to 6 months, the dilution can kill 60% to 70% of the leaf area. This indicates that methyl 4-hydroxybenzoate is effective as a pesticide for controlling A. adenophora. Observations show that within 30 minutes after spraying, the top buds, young leaves, tender stems, and some mature leaves begin to darken, wilt within 2 hours, and die within 4 hours. We examined the changes in six hormones (IAA, ABA, IP, JA, SA and TZ) during this process and found that IAA and ABA levels in A. adenophora were both down-regulated 4 hours after spraying with methyl 4-hydroxyphenylacetate and 20% alcohol. The two hormones changed potentially uncertain to plant death.