Postoperative spinal patients remain a challenge for provision of postoperative analgesia. Patient-controlled intravenous analgesia (PCIA) is a major method in reducing the severe pain after the surgery in our institution, but some adverse effects prevent the use of adequate dosage opioids.This study was determined using the probit analysis to investigate the optimal dose of dexmedetomidine (DEX) infusion for postoperative analgesia combined with sufentanil (SUF) in spine surgery.The dose of DEX needed to produce satisfactory analgesia conditions following combination of 3.0 μg/kg SUF in PCIA pump, which was diluted to 250 mL with a 4 mL/h as background infusion. Patients were recruited with age 35 to 65 years. The satisfactory criteria of postoperative analgesia were determined with a average satisfaction level of pain control, sedation, self-satisfaction, and adverse effects, among others. The dose of DEX was determined using the modified Dixon's up-and-down method (0.5 μg/kg as a step size). The first patient was test at 3.0 μg/kg DEX. The patient was assessed at 6, 12, 36 hours, and termination of PCIA following the continuous infusion of DEX-SUF mixture in PCIA after surgery.Twenty-five patients were enrolled by predetermined criteria. The optimal dose of DEX required for satisfactory analgesic was 4.33 (SD, 0.38) μg/kg combined with 3.0 μg/kg SUF via a PCIA volume of 250 mL by background infusion of 4 mL/h. Using probit analysis, the ED50 of DEX was 4.12 μg/kg (95% confidence limits 3.74-4.52 μg/kg) for satisfactory postoperative analgesic in spine surgery, the ED95 of DEX was 4.85 μg/kg (95% confidence limits 4.48-7.13 μg/kg). There was no report of somnolence or respiratory depression, relevant bradycardia or hypotension, or over sedation in this study.The optimal dose of DEX was 4.33 (0.38) μg/kg combined with 3.0 μg/kg SUF diluted to 250 mL with a background infusion of 4 mL/h for satisfactory analgesic after spine surgery. From probit analysis, ED50 and ED95 of DEX were 4.12 μg/kg (95% confidence limits 3.74-4.52 μg/kg) and 4.85 μg.kg (95% confidence limits 4.48-7.13 μg/kg), respectively.