As one of the core components of XLPE power cable line, lead sealing layer plays an important role in ensuring the proper working status of the XLPE cable. Due to its rugged surface, conventional non-destructive detection technologies such as ultrasonic measure are difficult to be directly used on detecting void and impurity defects in the lead sealing layer. In order to improve the efficiency and accuracy of ultrasonic inspection on the defects inspection results for lead seal layers, this paper employs a flexible ultrasonic sensor probe, i.e., an array of ultrasonic transducers embedded by a silicone rubber water bladder, to detect internal defects in lead sealing layer of power cables. A 2-D finite element simulation model was established using COMSOL Multiphysics software to simulate the ultrasonic wave propagation process inside the lead sealing layer, by multi-physics field coupling calculations. By setting different numbers of ultrasonic transducer sensors, the factors affecting the flexible ultrasonic detection of defects inside the lead sealing layer were analyzed. The results show that the number of ultrasonic transducer sensors has a significant effect on the detection results. With the increase in the number of sensors, the strength and clarity of the detected echo signal are significantly improved, which in turn enhances the sensitivity and accuracy of defect detection. Therefore, optimizing the transducer design can not only improve the detection results, but also significantly increase the detection accuracy.