Objective To investigate the effects of different preoperative fasting water time on body temperature in preoperative children. Methods Divided 350 children into A, B and C group randomly and apply 4 hours preoperative fasting water, 4 hours fasting water added 250～500 ml fluid infusion and 2 hours fasting water respectively. Observe the condition of body temperature, thirst, fidget, crying and vomiting in these 3 groups. Results The incidence rate of preoperative fever in group A was significant higher than that of in the other two groups. The incidence rate of thirst, fidget and crying were significant higher in group A and B than those of in group C, while there was no significant difference among three groups in the incidence rate of vomiting during the course of operation. Conclusion It is a safe method of 8 hours preoperative fasting food and 2 hours fasting water for children, which can effective reduce the incidence rate of preoperative fever and then guarantee the operation.