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 molL-1 , 0.04～25 molL-1 and 0.04～25 molL-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 molL-1 , Cd2+ concentration of 2.8 molL-1 and Pb2+ concentration of 3 molL-1 , the joint toxicity toward germlings reached the maximum. When Hg2+ , Cd2+ and Pb2+ dissolving in seawater were at lowconcentrations, 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 germlingsmost, 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 Pb2should be controlled under 0.04 molL-1 to avoid their combined toxicity especially in the process of artificial seeding of S.zhangii Tseng et Lu.