Abstract: Sargassum hemiphyllum, a common species in coastal waters of Guangdong Province, was selected to study its physiological response to different concentrations of heavy metal (Cd²⁺) stress and its tolerance to Cd²⁺ stress in laboratory semi-static experiments, so as to provide a basis for remediation of heavy metal pollution in that area. The results show that chlorophyll a (Chl a), carotenoid (Car), soluble protein (SP), soluble sugar (SS) and antioxidant enzyme activity (SOD and CAT) of S. hemiphyllum at Cd²⁺ concentration of 0.1 mg∙L⁻¹had no significant difference with the control (P>0.05). The contents of Chl a, Car, SP, SS, SOD and CAT at ρ(Cd²⁺) of 0.5−12.5 mg∙L⁻¹, were significantly higher than those of the control (P<0.05); there was no significant change in MDA contents under Cd²⁺ concentration of 0.1−0.5 mg∙L⁻¹ (P>0.05), and significant increase at Cd²⁺ concentration of 2.5−12.5 mg∙L⁻¹ (P<0.05). Therefore, S. hemiphyllum had stronger stress resistance at Cd²⁺ concentration of ≤0.5 mg∙L⁻¹, while  excessive Cd²⁺ stress of >0.5 mg∙L⁻¹ will be harmful for its photosynthesis and antioxidant function.