Background: Titrating a low dose of non-depolarizing neuromuscular blockers for induction of general anesthesia for patients with myasthenia gravis (MG) is a popular accepted practice. This practice may confront with unsatisfactory intubation conditions, especially for double lumen tube (DLT). This double-blind randomised study was to investigate whether magnesium sulfate could decrease the rocuronium demand and improve the DLT placement conditions for MG patients who scheduled for video-assisted thoracoscopic (VATS) thymectomy. Methods: The patients were randomly assigned into the magnesium sulfate or normal saline group. After acquiring the adequate sedative depth, magnesium sulfate(60mg.kg-1) or normal saline(control) was given to the patients in two groups respectively. After that, the same Train of Four (TOF) value was obtained with one or more doses of 0.05 mg.kg-1 rocuronium with 3 minutes interval, then DLT was intubated. The primary and secondary observational outcomes were the cumulative rocuronium dose and the intubation condition for DLT placement respectively. Results: 23 patients in the magnesium sulphate group and 22 patients in the control group completed the study. The amount of rocuronium used to meet the setting value of TOF were 0.10 (0.05)mg.kg-1 and 0.28(0.17) mg.kg-1 in the magnesium sulphate and control group respectively (P<0.0001). Under a similar depth of neuromuscular blockade and sedation, 23 patients in magnesium sulphate group and 16 patients in control group showed excellent intubation condition. The patients in both groups showed similar characteristics of recovery from neuromuscular blockade. Conclusions: Our research showed that magnesium sulfate could decrease the rocuronium demand for satisfactory DLT intubation condition in MG patients.