The detection of the interface between bored pile concrete and mud has been a bothering issue in construction, manual measurement error increases with the depth increasing of the empty hole section. Based on the physical properties that the density and electrical impedance of concrete are different with those of other media, an automatic detecting instrument of concrete interface for bored pile was developed. The paper introduces this instrument about its main structure composition, software design and the field test. The test result shows its good economic and social benefits.