In order to identify the impact echo response characteristics of two kinds of void-existed defects induced by the interface debonding between the grouting medium and duct's inner surface due to the shrinkage of grouting material and initial void in grouting medium within the duct of post-tensioned concrete members, four grouted specimens with different interface debonding state were fabricated by using interfacial bonding agent, expansive agent and curing ways after grouting, and another specimen with void defect in grouting medium was formed by presetting a foam in the duct. The spectrum response was continuously measured by impact echo system within five days after grouting. The results show that the spectrum within first 2 days after grouting of well grouted specimens without any preset void in the grouting medium were almost identical and similar to that of solid specimens without duct, and obvious interface debonding in specimens without expanding agent in the grouting material was observed, resulting in the lower thickness frequency and more complicated spectrum at the third and following days after grouting, which can be considered the principle to ascertain the occurrence of interface debonding for specimens with well grouted duct. In the specimen with a preset void in the grouting medium but without any interface debonding, although its thickness frequency was always lower than that of well grouted specimens without interface debonding, its spectrum form was very similar to that of the latter and its thickness frequency ratio almost kept unchanged as about 0.85. The thickness frequency development curves with the grouting age between the specimens with a preset void and interface debonding intersected, and both had the same frequency at about the third day, which indicates that the two different defects of interface debonding and existing void cannot be identified uniquely only by the thickness frequency at some moment. To distinguish these two defects mentioned previously through the response characteristics and the development curves of thickness frequency during about 4 days after grouting is necessary.