Abstract: Effect of self-allelopathy on AOS of Casuarina equisetifolia Forst seedling was studied using a nutritional cultivation method.The water extract of the roots from 15-year C.equisetifolia Forst was used for the self-allelopathy.The results indicated that the AOS lost its original balance,and that the AOS greatly accumulated in C.equisetifolia Forst under self-allelopathy.The permeability of plasma membrane increased,while the membrane structure and function were damaged;the MDA content and the membrane lipid peroxidation increased;the O2- generation rate and H2O2 in leaf cells increased;the AOS became imbalanced;AOS accumulated greatly in leaf cells;the AOS defense system was damaged;the activities of SOD,CAT,GR and POD decreased;and,the ability of cells in clearing O2- and H2O2 was diminishing.