Objective To investigate the maximum dose of continuous mivacurium infusion for intraoperative neuromonitoring (IONM) and observe the adverse reactions during thyroid surgery under total intravenous anesthesia (TIVA). Methods Thirty patients undergoing IONM during thyroid surgery received continuous infusion of mivacurium at the initial rate of 14.97 μg · kg-1 · min-1 . The infusion rate was adjusted in the next patient based on the response of the previous patient in IONM. The depth of anesthesia was maintained with propofol and remifentanil during the surgery. The EC50 and 95% CI of mivacurium were calculated with Brownlee's up- and-down sequential method. During the operation, body movement and skin flushing of patient was monitored, and the mean arterial pressure (MAP) and heart rate (HP) were recorded immediately (T0) and at 5 min (T1) after injection of muscle relaxant for anesthesia induction, immediately (T2) and at 10 min (T3) and 20 min (T4) after initiation of intraoperative infusion of the muscle relaxant. Results The EC50 for continuous infusion of mivacurium without affecting IONM was 18.9 μg · kg- 1 · min- 1 (95%CI: 17.3-20.5 μg · kg- 1 · min- 1 ) during thyroid surgery under TIVA. One patient (3.3%) developed transient facial skin redness after induction. Intubation difficulties or body motions occurred in none of the patients during the surgery. Pair-wise comparison showed no significant variations in MAP or HR of the patients at the 5 time points (P>0.05). Conclusion In patients undergoing thyroid surgery under TIVA, the EC50 for continuous infusion of mivacurium is 18.9 μg · kg- 1 · min- 1 (95%CI: 17.3-20.5 μg · kg- 1 · min- 1 ), which does not affect IONM or causes serious adverse reactions during the operation.