Abstract: The traditional piezoelectric ceramic ultrasonic sensors face the challenge of acoustic impedance mismatch at the interface with cable surfaces, whereas flexible piezoelectric ultrasonic sensors overcome this limitation and enable the detection of ultrasonic signals from partial discharge in cables. However, continuously enhancing the sensitivity of flexible piezoelectric ultrasonic sensors and further reducing their detection limits for ultrasonic signals are critical for their practical application. To achieve this goal, we focus on the optimization design of the sensitive material, namely, piezoelectric thin films, in flexible piezoelectric ultrasonic sensors, and investigate the influences of thickness on the comprehensive performance of Pb(Zr_0.52, Ti_0.48)O₃ (PZT) piezoelectric thin films and the sensitivity of the sensors fabricated with these films. The findings reveal that, due to the effect of interfacial dead layers, PZT piezoelectric thin films with a thickness of 1.2 μm exhibit a higher proportion of (100)-oriented grains and greater remnant polarization compared to films with thicknesses of 0.8 μm, 1.0 μm, and 1.6 μm, thus the PZT piezoelectric thin films possess the highest piezoelectric coefficient, highlighting the greater potential for practical applications. Flexible piezoelectric ultrasonic sensors fabricated using the 1.2 μm-thick PZT piezoelectric thin film were employed to detect ultrasonic signals generated by the fracture of mechanical pencil leads propagating in a 110 kV full-scale cable. The attenuation coefficient of the ultrasonic signal in the cross-linked polyethylene insulation layer was calculated to be 3.74 dB/m. This study provides a method to enhance the sensitivity of flexible piezoelectric ultrasonic sensors by optimizing the thickness of piezoelectric thin films, and offers both experimental and theoretical foundations for advancing the application of flexible piezoelectric ultrasonic sensors in cables and other electrical power equipment.