Abstract: With a view to provide some useful information for the control or kill the harmful micro-organism in the cultivation of Sipunculus nudus, the acute toxicity experiment of trichlorfon, fenpropathin and phoxim-fenvalerate mix on 2~3 cm body length nymph of S.nudus was performed in stilled water at the conditions of water temperature of 29.1~30.7℃, pH 8.0~8.2 and salinity of 29.5. The results indicated that the 24 h LC50, 48 h LC50 and SC (safe concentration) of trichlorfon were 1.03, 0.67 and 0.09 mgL-1. The 24 h LC50, 48 h LC50 and SC of fenpropathin were 0.70, 0.54 and 0.10 mgL-1. The 24 h LC50, 48 h LC50 and SC of phoxim-fenvalerate mix were 0.72, 0.48 and 0.06 mgL-1.