Objective To evaluate the predictive value of the variation of inferior vena cava for difficult weaning patients with cardiac dysfuncion.Methods Ninety-nine patients with cardiac dysfuncion were enrolled for prospective study.All patients were expected to weaned when they passed the screening from January 2014 to December 2015.Patients received SBT under PSV for 1 h.At the end of the SBT, the inferior vena cava was visualized under B ultrasound probe.The patients who passed the SBT and no need for either reintubation or noninvasive ventilation within 48 hours were include into successful weaning group, the others were included into difficult weaning group.Result Of the 99 patients admitted, 59 patients were successful weaning,40 patients were difficult weaning.A significant differences in the inferior vena cava between successful weaning group and difficult weaning group.It’s AUC to predict difficult weaning was 0.839,with a sensitivity of 69.5% and a specificity of 94.7%.There was a sensitivity of 91.7% and a specificity of 35.2% for determining difficult weaning when RBSI was ＞105,It’s AUC was 0.816 for RBSI.Conclusion The assessment of the variation of inferior vena cava using ultrasound was helpful to predict difficult weaning for patients with cardiac dysfuncion.