Abstract: In the explosive composite process of titanium-steel bar material, the shape of the joint surface is an important index to reflect the composite quality.The ultrasonic detection method of water immersion is adopted in this paper, and research on A-scan signal was performed. Through the short time Fourier transform of the second time frequency transform, the time-amplitude distribution non-stationary signal in the time domain was transformed into time-energy signal, and the ultrasonic image was established through displaying composite surface wave height and width dimensions.The results show that the imaging results are consistent with the actual composite size.It provides a new method for ultrasonic evaluation of explosive composite process of small diameter bar.