Abstract: The effect of water bath temperature on recovery rate of methamidophos during purification was studied. The samples of lettuce were heated and blow-dried at 80℃, 65℃, 55℃ and 45℃ respectively during the purification process, and the recovery rate of methamidophos was compared. The recovery rate of methamidophos was the highest at 45℃. Organophosphorus pesticides with poor thermal stability can properly reduce the temperature of water bath, improve the recovery rate and increase the accuracy of the experiment during the purification process of sample pretreatment.