Abstract: Abstract: [Goal]The test used the ethanol extract from Wedelia trilobata (L.)A.S.Hitche hairy roots of different concentration to give the seeds of Bidens pilosa L. etc a soak to find out the allelopathic effect, in order to lay the experimental and technological foundation for the production and application of bio-chemicals maded by Wedelia trilobata hairy roots under in vitro culture method. [Method]We Used the ethanol extract from Wedelia trilobata hairy roots in different concentration, 0.5g/mL and 1.0g/mL to give the seeds of Bidens pilosa L. etc a soak for different time period, 3 hours, 12 hours and 24 hours, and take the seeds soaked in water as a contrast. [Result] The higher concentration is or the longer the seeds soaked into the concentration, the lower the seed germination is. Besides, the seeds of Bidens pilosa L.、Digitaria ischaemum and D.sanguinalis、Setaria viridis(L.)Beauv. cannot germinate properly when soaked into the extract of low concentration, 0.5g/mL(fresh weight), for over 12 hours. And the inhibition effect is more obvious when the concentration of the extracts is 1.0g/mL(fresh weight), the seeds of Bidens pilosa L.、Digitaria ischaemum and D.sanguinalis、Echinochloa crusgalli (L.) Beauv.、Setaria viridis(L.)Beauv. And Herba taraxaci cannot germinate after being soaked into the high concentration extracts. What’s more, the inhibition effect is more obvious when the soak time is over 24 hours, no matter the concentration is 0.5g/mL or 1.0g/mL. [Conclusion] Wedelia trilobata hairy roots have strong allelopathic effect.