ObjectiveTo analyze the risk factors for post-thymectomy myasthenic crisis (PTMC) and prolonged mechanical ventilation, in myasthenia gravis patients who underwent extended thymectomy. MethodsWe retrospectively analyzed the clinical data of 79 patients including 38 males and 41 females who experienced PTMC and required mechanical ventilation in Daping Hospital between June 2008 and November 2014. Single factor analysis and multivariate analysis were conducted. ResultsMorbidity of PTMC was 20.6% (79/384). Result of single-factor analysis showed that postoperative pneumonia was one of the main reasons of prolonged mechanical ventilation (P < 0.05). Result of multiple-factor analysis showed that the operation time was positively correlated with mechanical ventilation time (P < 0.05). The risk factor of prolonged mechanical ventilation time in PTMC was not associated with sex, age, disease history, myasthenic crisis history, Osserman classification, dosage of pyridostigmine before and after the operation, surgical approach, bleeding volume, other therapies besides mechanical ventilation (P > 0.05). ConclusionMechanical ventilation is one the main therapy of PTMC, operation time, and postoperative pneumonia are the main factors to prolong mechanical ventilation time. In order to decrease morbidity of PTMC and shorten mechanical ventilation time, the operation time should be controlled and pulmonary infection should be avoided.