This paper analyzes two mal-operation examples of actual power plants differential protection. It draws a conclusion that the reason of mal-operation by non-synchronization closing and mal-operation by power consumption of auxiliaries quick-tripping， is not the non-synchronization closing inrush current caused by the transformer voltage changing more during synchronization closing， but the current transformer saturation caused by the non-periodic component of non-synchronization closing current. Some solutions are proposed also.