Background: Postoperative nausea and vomiting (PONV) is the most common and undesirable complications associated with anaesthesia, leading to discomfort in patients and extended hospital stays. The present study evaluates and compares the effects of preoperative low-dose mannitol infusion on PONV in patients undergoing gynaecological laparoscopic surgery.Methods: Fifty-three patients were randomly allocated into 2 groups (mannitol group, Group M, and control group, Group C). In the group M, low dose of 20% mannitol (0.5g/kg) was applied 30 minutes before induction. In the group C, equal volume of 0.9% normal saline was infused. Both groups were maintained with Ringer’s lactate in 8 ml/kg/h intraoperatively. The primary endpoint was the incidence and severity of PONV within the first postoperative 24h. The secondary endpoints were the average vascular resistance indexes(RI) and angle-corrected time-averaged flow velocity(TIMV)of bilateral vertebral artery (VA) and internal carotid artery (ICA) and cerebral blood flow volume (CBFV) using non-invasive Doppler ultrasound at time points just 5 min after laryngeal mask airway placement (T1：baseline);pneumoperitoneum started and position changed(T2);15 minutes after pneumoperitoneum with the position change (T3); pneumoperitoneum stopped with a position change (T4);15 minutes after pneumoperitoneum had stopped (T5).Results: Within 24h after operation, the incidence of PONV in group M (28.0%) was significantly lower than group C (56.0%) (P=0.045). Compared with group C, the severity of PONV in group M was significantly lower(P=0.040). The RI of VA and ICA were statistically different between the two groups(P<0.05).No significant differences were observed in the VM of VA and ICA and CBFV between the two groups(P＞0.05).Conclusion: Pretreatment with a low dose of mannitol effectively reduced the incidence and severity of PONV in patients undergoing laparoscopic gynecological surgery by reducing the RI of the VA and ICA.Trial registration: Chinese Clinical Trial Registry (ChiCTR-IPR-16007749, registered date: 12/01/2016.), http://www.chictr.org.cn