Abstract: In this study, chronic toxicity experiments were conducted to explore the biotoxicity of seawater with different Hg²⁺, Cd²⁺ and Pb²⁺ concentrations to germlings of Sargassum zhangii Tseng et Lu. For single factor experiment, with increasing heavy metal ion concentration in seawater, the toxicity of seawater to germlings enhanced significantly and accumulated with time when Hg²⁺, Cd²⁺ and Pb²⁺ concentrations in seawater were 0.04~1 μmol · L^-1, 0.04~25 μmol · L^-1 and 0.04~25 μmol · L^-1. The descending order of toxicity of three heavy metal ions to germlings was Hg²⁺>Cd²⁺>Pb²⁺. For orthogonal experiment, at the Hg²⁺ concentration of 0.08 μmol · L^-1, Cd²⁺ concentration of 2.8 μmol · L^-1 and Pb²⁺ concentration of 3 μmol · L^-1, the joint toxicity toward germlings reached the maximum. When Hg²⁺, Cd²⁺ and Pb²⁺ dissolving in seawater were at low concentrations, Pb²⁺ inhibited the growth of germlings most, followed by Cd²⁺. On the contrary, when Hg²⁺, Cd²⁺ and Pb²⁺ dissolving in seawater were at high concentrations, Hg²⁺ inhibited the growth of germlings most, followed by Cd²⁺. Moreover, when three metal ions worked together in seawater, Hg²⁺ and Cd²⁺ demonstrated synergistic effects whereas these two ions demonstrated antagonistic effect on Pb²⁺. The study suggests that the contents of Hg²⁺, Cd²⁺ and Pb²⁺ should be controlled under 0.04 μmol · L^-1 to avoid their combined toxicity especially in the process of artificial seeding of S. zhangii Tseng et Lu.