Abstract: Three water solutions with different trophic status (distilled water, BG11 nutrient solution and eutrophic water) were used to study the allelopathic effects of living and dead plants of the invasive species Alternanthera philoxeroides at different growth phases on Microcystis aeruginosa in a hydroponics experiment. pH and ORP (oxidation-reduction potential) were measured in cultivation solutions from living plants and decomposed solutions from dead plants of A. philoxeroides. 1) Under different trophic status of culture media, A. philoxeroides had an inhibitory potential and living and dead plants at different stages could significantly inhibit M. aeruginosa at the lag phase. Dead plants had a stronger inhibitory effect on M. aeruginosa than living plants. Living plants had allelopathic effects, in the order: distilled water＞eutrophic water＞BG11 nutrient solution, while for dead plants the order was: eutrophic water＞BG11 nutrient solution＞distilled water. 2) The two kinds of plant materials gave similar allelopathic inhibition of M. aeruginosa. The inhibitory effect initially increased (for all three trophic water bodies), and then terminated at a steady state. The highest inhibitory rates of living and dead plants at 70.7% and 91.7%, respectively, were at the 5th day. 3) The physical-chemical properties of exudates from living plants and decomposed dead plants were notably different, and pH values and ORP were related to various factors such as duration and types of solutions and materials. These findings will provide theoretical references for utilization and management of invasive plants and its further application in water-bloom algal control.