A short time high power pulse is stimulated by the LC circuit. Through a send winding, the electromagnetic guided wave signals transmit in the metal pipes. A receive winding is mounted around the pipe to collect the receive signals. The receive signals are different if there are several defects in the metal pipe or not. To analyze the instantaneous pulse group, a method of multi-scale entropy is adopted. The receive signals are transformed to a data sequence. In terms of the entropy values of the sequence at different scales, the complexity of the data sequence is gained. The defect information is contained in the entropy value varied curve.