Abstract Theoretical analysis for bonding evaluation by ultrasonic reflection characteristics is carried out with respect to FRP-to-FRP (Fiber Reinforced Polymer) bonded joints with thick adhesive layers. Spectrums of reflection coefficients depending on frequencies and incident angles are obtained, which are calculated theoretically and validated by finite element simulation. Meanwhile, in order to study the effect of interface degradation on reflection characteristics, variations of interfacial stiffness for single/dual bonding interfaces are investigated. It’s found that local extreme value points in frequency spectrums intend to shift to the lower frequency side with the decrease of interfacial stiffness. However, in angular spectrums, local extreme value points will shift to the larger angle side with the decrease of interfacial stiffness. The results also show that the local minima in both spectrums are closely related to the symmetry of the bonded joints, which is connected with the thicknesses of adherend layers, material properties, and the bonding states of dual adherend-adhesive interfaces.