Background Weaning-induced pulmonary edema (WIPO) is one of the commonest causes of weaning failure in critical care. As ultrasound machine is an easy and available tool in critical care units that help to diagnose interstitial lung syndrome by detecting B lines, it can be a handy method for diagnosis of WIPO. Aim We aimed to detect the threshold of increase in B lines (ΔB lines) that can accurately diagnose WIPO. Methods This is a prospective observational study carried on 52 mechanically ventilated patients who were eligible for weaning. Lung ultrasound was done before and after spontaneous breathing trial (SBT) to detect the increase in B lines (ΔB lines). Results Among the first 52 SBT, 25 trials were failed. Twelve patients developed WIPO (23% of all SBT and 48% of failing SBT). The best diagnostic accuracy for WIPO was a ΔB lines ≥ 6. In these cases, a ΔB line ≥ 6 has a sensitivity of 83.3%, a specificity of 82.5%, a positive predictive value of 58.8%, and a negative predictive value of 94.3% (area under the curve = 0.824, p < 0.001). Low body mass index (BMI) and increased number of ΔB lines were independent predictors of WIPO. Conclusion A ΔB lines ≥ 6 has the best diagnostic accuracy for diagnosis of WIPO during SBT. Trial registration NCT04463303.