To investigate the impact of body mass index (BMI) on the analgesic effects and adverse reactions of patient-controlled intravenous analgesia (PCIA). From 2017 to 2018, 390 patients undergoing open gastrointestinal surgery were reviewed at West China Hospital, Sichuan University. All used PCIA of sufentanil combined with dexmedetomidine and flurbiprofen axetil. According to their BMIs, they were placed into six groups: group A (BMI < 18.5kg/m2, 29), group B (18.5kg/m2 ≤ BMI< 22kg/m2, 124), group C (22kg/m2 ≤ BMI < 24kg/m2, 99), group D (24kg/m2 ≤ BMI < 26kg/m2, 69), group E (26kg/m2 ≤ BMI < 28kg/m2, 46) and group F (BMI ≥28kg/m2, 23). Main data of the perioperative use of analgesics, postoperative visual analogue score (VAS), and adverse reactions were collected. Twenty-four hours (h) after surgery, patients in group A had a higher resting VAS than the other groups, especially B (pA-B = 0.011). VAS of patients during activity in group B was lower than those in group C 48 h after surgery (p = 0.013). Compared with groups B to F, group A had a significantly lower incidence of hypertension (p = 0.012) and a significantly higher incidence of vomiting 24 h after surgery (p = 0.009). Binary logistic analysis found that higher age was a risk factor for vomiting 24 h after surgery (OR 1.158, p = 0.045). Using the same PCIA, patients with BMIs of less than 18.5 kg/m2 had worse analgesia on the first day after surgery and were more likely to vomit. Postoperative analgesia and related experiences in patients with BMIs of less than 18.5 kg/m2 need to be improved. © 2020 Li et al.