Abstract: In order to discover the action mechanism of acoustic scattering in synthetic aperture array imaging of ultrasonic guided waves in plate with welding structures, the scattering amplitude matrix (SAM) and the scattering flight time matrix (SFTM) were proposed to analyze and describe the acoustic scattering of ultrasonic shear horizontal(SH) guided waves, the action relationship between the weld scattering and the boundary scattering were researched emphatically. The research result shows that, the scattering matrix may be a forceful analysis tool that may be used into discovering the action mechanism of acoustic scattering in synthetic aperture array imaging of ultrasonic guided waves, the SAM and the SFTM have the same scattering matrix factor that lead to pseudo strip-shaped shadows with the same degree and the characteristic of elliptical cluster tracks in array image, and then the background noise of image may increase and signal-nose ratio may decrease accordingly. The research work proposed would provide support for no-line array imaging algorithm of ultrasonic guided waves in plates with welded structure.