The algae liquid EC value increased obviously in the high-concentration P. stratiotes root exudate treatment groups. The EC values of the 0.8 and 1.2 g mL−1 treatment groups on the eighth day were 3.7 and 4.1 times greater than that of the control group, respectively, indicating that M. aeruginosa algae cell membrane structure was damaged under the stress of high concentration of allelochemicals. Increasing the cell membrane permeability caused the ions inside the cell to be discharged to the outside, resulting in the increase of EC value.