Vasoplegic syndrome (VS), also known as vasodilatory shock, is a type of man ifestation characterized by severe hypotension accompanied by high hemodynamic output and low resistance that often occurs early after cardiac surgery under cardiopulmonary bypass. If this complication occurs after cardiopulmonary bypass, it will cause irreversible multi-organ function damage to patients if not treated in time. This case describes an 8-month-old girl who underwent repair of ventricular septal defect under cardiopulmonary bypass,and VS occurred during the operation. The treatment by intravenous methylene blue was effective and the prognosis was good.