Abstract: Because of the acoustic scattering from the coarse grain structure, it is not easy to identify defect wave when ultrasonic pulse echo method is applied in testing austenitic stainless steel weldment.In this paper, an improved de-noising method based on Wavelet packet is proposed. The principle of the improved technique is described. The ultrasonic echo containing the artificial defect wave in the austenitic stainless steel weldment is collected. Using the proposed method, the ultrasonic echo is de-noising processed. Then the artificial defects are quantitative measured. The results show that using the method, the noise component in the ultrasonic echo can be effectively suppressed, and the flat bottom hole of 1.5mm in diameter can be identified.