For　composite　bonded　joints　with　thick　adhesive　layers,　the　finite　element　method　and　non-contact　ultrasonic　testing　technology　are　applied　to　analyze　the　ultrasonic　reflection　characteristics　when　bonding　interfaces　are　degraded.　Firstly,　the　time　domain　simulation　model　of　ultrasonic　reflection　characteristics　for　fiber-reinforced　polymer　(FRP)　plates　under　water　immersion　is　established.　The　variation　rule　of　the　acoustic　reflection　coefficient　at　different　incidence　angles　is　numerically　analyzed,　and　compared　with　the　published　theoretical　methods.　Then,　the　time-domain　simulation　model　of　reflection　characteristics　of　the　composite　bonding　structure　with　the　thick　adhesive　layer　is　built.　And　the　effects　of　interface　weakening　on　ultrasonic　reflection　characteristics　of　bonded　structures　are　analyzed　by　changing　the　normal　spring　stiffness.　Likewise,　the　influence　of　the　weakening　degree　of　bonding　interface　on　spectral　characteristic　points　of　ultrasonic　reflection　coefficient　is　analyzed　in　detail.　Based　on　the　water　immersion　ultrasonic　measurement　system,　the　frequency　spectrum　of　ultrasonic　reflection　coefficient　for　normal　bonding　states　is　in　good　agreement　with　the　calculation　results　of　the　finite　element　method.　In　addition,　FRP　bonding　structures　with　different　bonding　interface　states　are　prepared.　The　relationship　between　the　ultrasonic　reflection　characteristics　in　the　bonding　structure　and　the　bonding　quality　is　analyzed,　which　is　consistent　with　the　simulation　data.　Based　on　the　regular　distribution　characteristics　between　different　bonding　states　and　ultrasonic　reflection　properties,　it　provides　a　research　idea　for　the　bonding　quality　assessment　of　the　composite　bonded　structures.　©　2024　Science　Press.　All　rights　reserved.