In order to explore the special mechanism of phosphorodithioate, an organophosphorus pesticide, on Nile tilapia (Oreochromis niloticus), Nile tilapia and Mud carp (Cirrhinus molitorella) were selected as materials to study their living conditions and activity change of AChE under various concentration poison exposure and duration. The results showed that, in acute toxicity test and recovery in water test, the mortality of Nile tilapia and Mud carp exposed to 0.05 and 0.1 mg/L solution were 100% and 0 respectively, and the activity of AChE in brain of Nile tilapia and Mud carp were significantly decreased with the increase of concentration and exposure time (P < 0.05), but the drop rate in Nile tilapia was much greater than that in Mud carp, and this kind of decrease was irreversible; The activity of AChE in brain of Mud carp, however, renewed to 96.25% and 79.33% after ten days recovery in clean water. Hence, it could be concluded that the phosphorodithioate had high toxicity on Nile tilapia and had no obvious toxicity on Mud carp at a lower concentration; the selective poison mechanism of two species might be related with the different sensitivities of the activity of AChE on phosphorodithioate.