Abstract: Ambrosia trifida is a widespread weed of compositae in the Northeast of China and its allelopathic effects were studied in order to elucidate its spreading mechanism. Firstly, the allelopathic effects of different parts of A. trifida were investigated, and some interesting results were found out. 1)The water extract of shoot had significant inhibitive effects on crop seed (soybean, maize. wheat and rice)germmation and primary growth, and changed the crop root morpha. The most allergic species were soybean and maize. 2)The water ex tract of root had no effect on seed germination, but had a different effect on crop primary growth The extract could inhibit root elongation of wheat by 34% while it stimulated bud growth by 31%. 3)Volatile and soil had no effects on the crop seed germination and growth Sceondly, the GC and GC/MS analysis of water extract in A.trifida showed that α-pinene, β-pinene, cineole. camphene, spanthueol might be the potential all elochemicals in its water extract. Results obtained in this study indicate that the allelopathic effecls might play an important role in the spreading of the weed.