A　novel　method　is　proposed　for　quantitative　characterization　of　cracks　by　laser　ultrasound.　It　is　found　that　cracks　with　different　depth　will　have　different　feedbacks　to　sound　waves　based　on　crack　closure　effect,　energy　loss　theory　and　resonance　phenomenon.　Therefore,　damage　sensitive　characteristic　parameters　can　be　extracted　from　the　acoustic　waves　and　used　to　quantitatively　characterize　the　crack　size,　including　the　waveform　duration,　the　reflection　energy　coefficient　and　frequency　spectral　area.　In　this　paper,　cracks　with　gradient　depth　are　used　as　experimental　objects,　and　the　relevant　data　is　obtained　through　simulation　and　experiments.　The　defects　are　positioned,　and　the　positioning　error　is　less　than　3%.　The　study　found　that　as　the　crack　depth　increases,　the　waveform　duration　and　the　reflection　energy　coefficient　gradually　decrease,　and　the　frequency　spectral　area　first　increases　and　then　decreases.　When　the　crack　depth　is　larger　than　0.9mm,　characteristic　parameters　tend　to　be　fixed　values.　When　the　crack　depth　is　0.5mm-0.7mm,　abrupt　phenomena　occurred　in　both　the　reflection　energy　coefficient　and　the　frequency　spectrum　area　due　to　resonance　phenomenon.　By　analyzing　the　corresponding　relationship　between　damage　sensitivity　characteristic　parameters　and　depth,　the　ability　of　laser　ultrasonic　quantitative　measurement　can　be　improved.　©　2020　Elsevier　GmbH