During the manufacturing process of honeycomb sandwich structure formed by co-curing, the absence of fillet styrofoam acts as a common defect type. Generally, X-ray detection is used as a major detection method. In this research, an ultrasonic detection method based on focused water-bag probe is adopted. The foregoing probe is equipped with an integrated rubber water bag, which can achieve soft contact with the detection surface and completely fit the detection surface. In testing, the probe slides along the length direction of the R region to scan a specimen containing artificial defects. Experimental results indicate that the proposed method can effectively present the characteristic signals of defects, thereby realizing accurate identification of such defects.