The allelochemicals released by Eupatorium adenophorum roots is an important factor inducing the changes of the weed soil environment, which provides favorable conditions for the successful invasion of the weed. By using GC/MS technique, the components and their relative contents of phenolic allelochemicals in aphid-infested and non-infested E. adenophorum root zone soils were analyzed, with unplanted soil as the control. Less difference was observed in the components of phenolic allelochemicals among the test soils, but their relative contents differed significantly. The relative contents of benzoic acid and 4-hydroxybenzoic acid in aphid-infested E. adenophorum root zone soil were significantly higher than the control; whereas that of 4-hydroxylcinnamic acid was in reverse. In the test three soils, the proportions of benzoic acid, 4-hydroxybenzoic acid, and 4-hydroxylcinnamic acid were different. The bacteriostatic test with the three acids and their combinations based on their proportions in test soils showed that at lower concentrations (50-150 mg x L(-1)), the three acids had significant inhibitory effects on five kinds of soil-borne pathogens, but their combinations had different inhibitory effects on the five pathogens, with the effects being significantly higher for the combinations with the proportions in non-infested E. adenophorum root zone soil and the control.