Alternanthera philoxeroides is a perennial invasive species worldwide which can greatly affect native ecosystems and agricultural production. Our research studied the allelopathic effects of aqueous extracts of A. philoxeroides on the growth and antioxidant enzyme activities of Zoysia matrella, and isolated and analyzed the dominant allelochemicals in root extracts of A. philoxeroides. The overall allelopathic effects of A. philoxeroides extracts on the growth and antioxidant enzyme activities of Z. matrella were found to be slightly stimulatory (concentrations ≤10 g L^-1) and highly inhibitory (≥40 g L^-1). Malondialdehyde contents were significantly enhanced with increasing concentrations of A. philoxeroides extracts. The strength of the allelopathic effects of three extracts of A. philoxeroides on Z. matrella followed the order: roots > leaves > stems. The dominant substance was extracted and identified to be ethyl propionate by gas chromatographymass spectrometry (GC-MS). Pot experiment results show that the effects of ethyl propionate on growth and enzyme activities of Z. matrella also ranged from slightly stimulating to highly inhibiting in a similar fashion to the effects of A. philoxeroide extracts on Z. matrella. The overall allelopathic effects of A. philoxeroides on Z. matrella ranged from slightly stimulating to highly inhibiting. The most abundant allelochemical component of root extracts was identified as ethyl propionate, which also exhibited inhibitory effects similar to A. philoxeroides extracts on Z. matrella.