Abstract: Using static bioassay test, the acute toxicity of Mn2+ and Mn7+ on behavior and survival of juvenile sea cucumber Apostichopus japonicas was studied, furthermore, the accumulation in Ap. japonicas was also analyzed. The results showed that at 96 h of exposure, the mortality rates of Ap. japonicas were 0.0 % in all groups of Mn2+ treatment, no significant acute toxicity of Mn2+ was found within the experimental concentration range of 0.8 ~ 16 mg/L; However, when exposed to Mn7+ within the range of 0.1 ~ 6.5 mg/L, the mortality rate increased with the Mn7+ concentration and exposure time. At 48 h of exposure, the mortality rate of larval Ap. japonicas showed a significant positive correlation with Mn7+ concentration (P 0.01). Mn7+ at concentrations higher than 0.5 mg/L showed the significant acute toxicity, and its safety concentration (SC) for the juvenile sea cucumber was 0.31 mg/L. Therefore, Mn7+ had the stronger acute toxicity than Mn2+ to Ap. japonicas. With increasing concentrations of Mn2+ and Mn7+, the Mn content and accumulation rates in juvenile bodies gradually increased, but changes in bioconcentration factor differed between Mn2+ and Mn7+.