Objective To observe the value of ultrasound assessment of diaphragm function combined with the Medical Research Council(MRC)muscle strength score and acute physiology and chronic health Ⅱ(APACHE Ⅱ)score for predicting weaning outcome in severe pneumonia patients after mechanical ventilation.Methods Totally 102 patients with severe pneumonia who underwent mechanical ventilation were retrospectively enrolled.Spontaneous breathing trial(SBT)was performed,and diaphragmatic mobility and contraction velocity were measured before and after SBT to evaluate diaphragm function.All patients were divided into successful weaning(successful group,n=76)and failed weaning group(failure group,n=26)according to breathing condition after weaning.Univariate analysis and multivariate logistic regression were used to analyze clinical data,MRC and APACHE Ⅱ scores before SBT,as well as diaphragm function parameters before and after SBT,and the independent risk factors for weaning failure were screened,based which a combined regression model was established.The diagnostic efficacy of each independent risk factor and combined regression model was evaluated by receiver operating characteristic curve and the area under the curve(AUC).Results MRC and APACHE Ⅱ scores,diaphragmatic mobility and contraction velocity before SBT were all independent risk factors for weaning failure in severe pneumonia patients after mechanical ventilation(all P＜0.05),with AUC of 0.682,0.667,0.785 and 0.685,respectively.The AUC of combined regression model established with the above independent risk factors was 0.898,higher than that of each independent risk factor(all P＜0.05).Conclusion Ultrasound assessment of diaphragm function combined with MRC muscle strength and APACHE Ⅱ scores could effectively predict weaning outcome in severe pneumonia patients who underwent mechanical ventilation.