Abstract: Ultrasonic phased array technology for testing deep, small and bad location defect is studied, and the characteristic signals and imagery related to defects are analyzed.In order to enhance signal to noise ratio and imagery resolution, the wavelet packet soft threshold denoising method with improved threshold is proposed. The results indicate that defect's signals are widely distributed in whole frequency domain, so that simple filtering in frequency domain on testing signal will lead to losses of defect's component inevitably. Wavelet packet denosing technique can be used to remove noise on the premise of keeping defect's component, through optimizing denosing threshold and choosing decomposed signals for composing, and wavelet denoise method proposed in the research is liable to enhance the signal and imagery quality of ultrasonic phased array testing on aero turbine disc.