Based　on　the　method　of　ultrasonic　normal　incidence,　the　relationship　between　reflection　coefficients　and　resonance　frequencies　under　the　boundary　condition　of　an　adhesive　layer　with　a　spring　model　is　analyzed.　The　stiffness　ratio　expressions　of　tangent　and　normal　are　derived　from　the　once　reflection　echo　of　the　adhesive　layer.　The　relationship　between　the　stiffness　ratio,　resonance　frequency　and　reflection　coefficient　of　adhesive　layer　under　multiple　reflection　echoes　is　derived,　and　the　variation　rule　of　reflection　coefficients　of　weak　bonded　structures　with　different　stiffness　ratios　is　observed.　Finally,　the　influence　on　the　resonance　frequency　caused　by　the　variation　of　thickness　of　a　weak　bonded　structure　is　analyzed.　The　theoretical　analysis　and　numerical　calculation　indicate　that　the　resonance　frequency　has　the　trend　of　shifting　to　a　higher　frequency　level　with　the　increasing　of　the　stiffness　ratio,　and　the　bonded　structure　has　multi-order　resonance　frequencies　with　the　increasing　of　the　adhesive　layer　thickness.