Abstract: In order to solve inverse problem of non-destructive weld defect testing, ultrasonic signal for a planar defect is simulated and the images are forward synthesized. Based on Multi-Gaussian Beams theory, ultrasound propagation behaviors are described. It includes in the probe wedge, wedge-testing piece interface and the testing piece. Based on these, an inspective model is established by employing Kirchhoff approximation theory. A bottom surface broken slot used is fabricated and the artificial planar defect is tested using ultrasonic method. Using the model, ultrasonic A-scan lines are simulated and D-, B-scan foreground images are synthesized. The experimental results show that the simulated dada are in better accordance with the measured ones. Forward synthesis can do help to optimize the testing parameter and solve inverse problem of non-destructive characterization.