In this study,effects on physiological and biochemical indexes of gametophytic blades of an low nitrogen and phosphorus tolerance line(9-Ⅳ) of Porphyra haitanensis under low nitrogen and phosphorus stress were observed.They were grown in medium with low content of nitrogen and phosphorus for 0,2,4,6,8 and 10 days.Changes in three quality indexes were measured,including the contents of pigment,crude protein and free amino acids.Results showed that all the three quality indexes of P.haitanensis were significantly declined under low nitrogen and phosphorus stress.Furthermore,changes in physiology also were measured,including free radical content,H₂O₂,malondialdehyde(MDA)and proline levels,and the activities of superoxide dismutase,peroxidase,and catalase.These physiological indexes were all presented as dynamic changes in the stress days.Based on these data,the physiological changes within the 9-Ⅳ blades were as follows:low nitrogen and phosphorus stress→cell membranes were damaged by oxidation and MDA levels increased→cells received signals of excessive ROS→antioxidation and osmoregulation systems began to clean out excessive ROS→free radical levels declined.Our results laid a foundation for understanding the principle of low nitrogen and phosphorus tolerance in P.haitanensis,and provided some directions for future breeding.