Abstract: Using the water extracts of the poplar roots to treat the seeds of Cassia tora (L.), and the water extracts of C.tora seeds to treat the poplar cuttings, the allelopathy between the two species was studied. The results showed that some allelopathy existed in poplar-Juemingzi agroforestry system. The water extraction of the roots of two poplar clones with 3—10 mg/mL concentrations could promote seed germination, seedling height, root growth and simplify the vigor index of C.tora, while the water extracts with 30 mg/mL concentrations suppressed seed germination and seedling growth. The root growth of poplar cuttings was improved under treatment with relatively low concentration of water extracts of C.tora seeds, while the root growth was suppressed at a high concentration. However, no significant difference in young seedling biomass was observed among the treatments. With increase of water extraction concentrations of C.tora seeds, the root vigor of poplar cuttings showed a hyperbola pattern, e.g., at the concentration of 3—12 mg/mL, the root vigor was improved, and reached the highest at 6 mg/mL. However, the root vigor was lowered at the concentration of 24 mg/mL.