The objective of this study was to detect water absorption of CFRP bonded joints, which is one of the causes of week bond. For this purpose, non-destructive evaluation method using the ultrasonic pulse technique was suggested. The amplitudes of the reflected waves when ultrasonic waves are incident on the adhesive joint are modeled, and it is shown that the intensity ratio of the surface reflected wave to the reflected wave from the adhesive layer is a function of the non-dimensional thickness, acoustic impedance, and attenuation ratio of the adhesive layer, and a method to identify these values is proposed. As a result of the experiment, it was shown that the amount of water absorption could be evaluated from the non-dimensional thickness of the adhesive layer, regardless of the base material CFRP laminated structure. On the other hand, from this experiment, clear relationship was not observed between the acoustic impedance and attenuation of the adhesive layer and the water absorption.