Insulation defects in high-voltage cable accessories easily induce partial discharge and even severe faults under sustained electrical stress, threatening cable line safe operation directly. However, existing defect detection methods are difficult to accurately identify early latent defects and are susceptible to electromagnetic interference. This study aims to clarify the influence law of insulation defects on ultrasonic characteristics, reveal the propagation mechanism of ultrasonic waves in insulating materials with defects, and establish a correlation relationship between ultrasonic characteristics and insulation performance. The research takes XLPE (cross-linked polyethylene), SIR (silicone rubber), and SEMI (semiconductive material) as objects, artificially prepares single-layer (XLPE, SIR) and double-layer composite (XLPE/SIR, XLPE/SEMI, SIR/SEMI) insulation samples with bubble defects. The experimental results show that samples with bubble defects produce obvious defect echoes, with amplitudes of 10% to 20% of the incident wave, and the ultrasonic amplitude attenuation of XLPE is greater than that of SIR. Defects cause the leakage current of insulating materials to increase by 67.22% to 81.49%, and the breakdown strength to decrease by 11.53% to 30.33%, which is closely related to the accumulation of charges at the defect site. The simulation results reveal that the semi-crystalline structure of XLPE enhances ultrasonic absorption attenuation, and the generation of defect echoes is due to the significant difference in acoustic impedance between the defect and the insulating material. The correlation relationship between ultrasonic characteristics and insulation performance in this study provides a theoretical basis for the early identification and state assessment of insulation defects in cable accessories.