Abstract: The toxic effect of different copper salts on juvenile sea cucumbers was studied by acute toxicity test. LC₅₀ and safety concentration of two copper ions (CuSO₄ and CuCl₂) were determined by statistical analysis of dead numbers of juvenile sea cucumbers (Apostichopus japonicus) after treatments of 24 h, 48 h, 72 h and 96 h, respectively. The toxic effect of different copper salts on foregut of juvenile sea cucumbers were observed on paraffin section with microscope. The results show that LC₅₀s of CuSO₄ on juvenile sea cucumbers after treatment of 24 h, 48 h and 72 h were higher than that of CuCl₂, while LC₅₀ of CuCl₂ was higher than that of CuSO₄ after 96 h treatment. The difference of toxic effect of juvenile sea cucumbers exposed to the two copper ions within 48 h was very significant, but was not significant as the exposure time. Corrosive actions were observed in digestive tracts of juveniles treated with two kinds of salts (with concentration of 0.06 mg · L^-1 as copper ion), which resulted in connective tissue layer separating from the epithelial layer. The toxicity (corrosive action) of CuCl₂ on digestive tracts of sea cucumbers was greater than that of CuSO₄. Different anions of copper salts had antagonistic effect to copper toxicity.