Abstract: AZ91 cast magnesium alloy with artificial defects in shape of hole were investigated to relate with the ultrasonic A signal and its characteristic value. The results indicated that there existed a good linear relation between the A signal amplitude and the diameter and position of the holes. Also, the research adopted low pass filter and wavelet analysis to decrease the noise, and the results indicated that using wavelet analysis the signal to noise ration could be effectively raised for the ultrasonic A signal of small holes.