Once　a　fire　occurs　in　a　cable　shaft,　it　is　easy　to　produce　the　＇chimney　effect＇,　thereby　accelerating　the　spread　of　fire.　In　order　to　evaluate　the　performance　of　the　fireproof　sealing　system　for　the　cable　shaft,　a　defect　detecting　method　for　the　fireproof　sealing　system　based　on　piezoelectric　ultrasonic　is　proposed　in　this　paper.　Firstly,　the　propagation　characteristics　of　Lamb　waves　in　composite　fireproof　board　are　analyzed,　and　the　frequency　of　excitation　signal　is　determined　by　frequency　sweep　experiments.　Secondly,　through　the　defect　detection　experiment　based　on　ultrasonic　guided　wave　on　the　composite　fireproof　board,　the　defect　detection　and　location　of　the　composite　fireproof　board　are　studied.　The　research　results　show　that　the　Lamb　waves　can　be　excited　by　a　piezoelectric　sensor　with　a　resonant　frequency　of　130　kHz　in　the　composite　fireproof　board,　and　it　can　be　received　by　a　piezoelectric　sensor　array.　By　comparing　the　attenuation　rates　of　the　received　signal　amplitudes　of　different　sensors,　the　defects　of　the　composite　fireproof　board　can　be　detected　and　located.　©　2022　IEEE.