Objective To investigate the risk factors and short-term outcome in very old patients undergoing mechanical ventilation. Methods A total of 270 very old patients undergoing mechanical ventilation in geriatric departments of Chinese PLA General Hospital from January 2008 to June 2013 were enrolled in this study. All the patients were divided into death group (n＝126) and survival group (n＝144) according to their outcomes in 28d, and from 29d to 3 months after mechanical ventilation. Univariate analysis was used to explore the related factors for prognosis from clinical data. Multivariate logistic regression analysis was carried out to determine the risk factors for death in these patients after mechanical ventilation. Results The cohort of patients was at the age of (89.0±4.8) years. Pneumonia (70.7%) was the major cause of mechanical ventilation. The other causes were acute heart failure (10.7%) and chronic obstructive pulmonary disease with acute exacerbation (AECOPD, 7.8%). There were 86 cases (31.9%) died within 28d after mechanical ventilation, and 126 cases (46.7%) died within 3 months. Multivariate logistic regression analysis revealed that low oxygenation index (PO2/FiO2), low serum pre-albumin, increased blood urea nitrogen (BUN), high serum creatinine (SCr＞165.2μmol/L), and using positive end-expiratory pressure (PEEP) were the prognostic factors in those patients who died within 28d after mechanical ventilation (all P＜0.05). Chronic kidney disease (CKD) and the level of C-reactive protein (CRP) were the prognostic factors in those patients died from 29d to 3 months after mechanical ventilation (P＜0.05). Conclusion Pneumonia, acute heart failure and AECOPD are common causes of mechanical ventilation in elderly patients. CKD, low PO2/FiO2, low serum pre-albumin, CRP, increased BUN, high Scr (＞165.2μmol/L) and using PEEP are the prognostic factors in the elderly patients with mechanical ventilation.