Abstract: The current study aimed to find the best anesthetic for accurate body length measurement. The anesthetic effects of magnesium sulfate, eugenol and ethylene glycol phenyl ether (EPH) on juvenile sea cucumber (Apostichopus japonicus) (1.3815±0.2138) g, (Mean±SD) were tested at different concentrations and treat times. The results showed that: (1) the anesthetic effect of EPH was minus and the eugenol had a moderate anesthetic effect only at low concentration (≤0.3 mol/L). Both reagents caused significant side effects at increased concentrations. The anesthetic effect of magnesium sulfate was best at the concentration of 0.3-0.4 mol/L. The anesthetic took effects about half an hour after treatment. The anesthetic and recovery rates all reached to 100%. (2) Individuals were treated by magnesium sulfate sea water solution at concentrations of 0.3 mol/L and 0.4 mol/L respectively. The body lengths were extremely significantly longer than those before treatment, and were not significantly different among 1 h, 1.5 h and 2 h treatments for both concentrations. These above results indicated that the best work concentration of magnesium sulfate for A. japonicus was 0.3-0.4 mol/L. The suggested concentration didn’t affect the growth of sea cucumbers and greatly improved the accuracy of body length measurement. Thus our suggested anesthetic concentration and treat time is 0.3-0.4 mol/L and 1 h respectively.