Abstract: When ultrasonic pulse reflection method is used to detect ceramic bonded parts, the echo signal will be weak due to the thickness of the specimen and the characteristics of the composite material, which is easy to be covered by the noise signal, and effective information cannot be extracted. To solve this problem, an improved threshold function noise reduction method was proposed. The experimental results showed that the proposed method could effectively improve the constant error of soft threshold function and the discontinuity of hard threshold function. Compared with the traditional method, the proposed method can process the defect signal of thick ceramic-based adhesives.It has applicability, which can not only effectively retain the ultrasonic defect signal of thick ceramic-based adhesives, but also has a better denoising effect, the signal-to-noise ratio of the denoised signal is increased by about 15%, the noise suppression ratio is increased by about 20%, and the root mean square error is minimal, which is feasible for practical application.