In ultrasonic testing, the quality of flaw echo-signals is a foundation for locating, quantitative and qualitative analysis. Thus, signal de-noising and increasing of the signal-to-noise ratio (SNR) are a key to successful application of ultrasonic NDT and NDE. In this paper, the theory of discrete wavelet transform (DWT) and stationary wavelet transform (SWT) is introduced. Moreover, differences are compared between DWT and SWT in signal decomposition and reconstruction. And according to the characteristics of ultrasonic flaw echo-signals, the principle and method of de-noising via thresholding based on SWT are studied. Finally, the experiment of signal de-noising by DWT and SWT are carried out for actual defect echo-signals. The experimental results show that SWT has the advantage of translation invariance after signal de-noising. This method not only can greatly remove the noises, but also provide the exact data for accurate localization of flaw.