Abstract: 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.