Abstract: We studied the effect of Cu²⁺ stress at different concentrations on the growth and physiological biochemical characteristics of Sargassum hemiphyllum by a semi-static test for 8 d. The results show that the relative growth rate (RGR) of thalli in the control increased initially and then decreased with time; on the 8^th day, the RGR of thalli was higher than that in the control only when Cu²⁺ concentration was 0.05 mg · L^-1. The physiological biochemical indices showed a fluctuation trend with time. On the 8^th day, these indices of thalli at Cu²⁺ concentration of 0.01 mg · L^-1 had no significant difference with the control; when Cu²⁺ concentration was 0.05 mg · L^-1, the catalase (CAT) activity and malondialdehyde (MDA) content were significantly higher than those in the control, and the other indices recovered to the control level. However, in Cu²⁺ > 0.05 mg · L^-1 groups, chlorophyll a (Chl-a), carotenoid (Car) and soluble protein (SP) were lower than those in the control; the other indices were higher than those in the control. Compared with 0.25 mg · L^-1 Cu²⁺ group, 1.25 mg · L^-1 Cu²⁺ group had more significant influence on biochemical indices except MDA. Therefore, appropriate concentration of Cu²⁺ (≤ 0.05 mg · L^-1) had positive effect on the growth and physiological biochemical indices of S.hemiphyllum, but there was negative impact on the growth and the ability of resilience of thalli when the concentration of Cu²⁺ was extremely high (> 0.05 mg · L^-1).