Abstract: At present, the internal defects of transformer sealing rubber lack convenient and effective non-destructive testing methods before installation. Under long-term and complex operating conditions, these defects may lead to failure, resulting in insulation oil leakage and compromising safety. This study proposes the application of phased array ultrasonic testing (PAUT) technology for detecting internal defects in transformer sealing rubber. First, the longitudinal wave transmission method was employed to measure the sound velocity and acoustic attenuation coefficient of the sealing rubber at different ultrasonic frequencies. Subsequently, PAUT process parameters were investigated using prefabricated nitrile rubber specimens containing internal defects, and the developed methodology was applied in engineering practice. The results indicate that both the sound velocity and acoustic attenuation coefficient of the sealing rubber increase with frequency, necessitating a balanced consideration of the influence of center frequency on PAUT detection performance. Dual-sided longitudinal wave phased array detection proved effective in identifying internal defects in transformer sealing rubber plates, achieving a sensitivity equivalent to a Φ1.5 mm flat-bottomed hole under the recommended process parameters. This research provides a useful reference for detecting internal defects in sealing rubber products with varying compositions, complex shapes, and special thicknesses.