In this paper, a debonding detection methodology for the interface between the lower surface of horizontal diaphragms and the concrete core of large scale irregular multi cellular concrete filled steel tubulars (MC CFSTs) with large cross section is proposed and experimentally studied by the use of the electro mechanical impedance (EMI) measurements of PZT materials. A number of PZT patches are pasted on the upper and lower surface of the horizontal diaphragms and embedded Piezo based Functional Elements (EPFEs) are embedded in the concrete. The EMI of both the PZT patches on the upper and lower surface of the diaphragm and the EPFEs in concrete core at different frequency regions are measured. The EMI measurements of PZT materials corresponding to the debonding areas show obvious variation. Results show that the proposed EMI based approach is capable of detecting the interface debonding defects for MC CFSTs.