when testing the defect by ultrasonic time of flight diffraction (TOFD), the image signal detected is often reshaped and resulted in inaccuracy of defect measurements because the probe beam has a limited range. This paper proposed an image enhancement method based on synthetic aperture focusing technique (SAFT). A mathematical model of testing image was first established based on the generation process of the image. A method was then designed for detecting defects in near surface blind area based on filtering of through waves. Finally, a synthetic aperture focusing algorithm based on peak capture pretreatment was used to enhance the resolution of the image. By applying the above method to test a crack defect of 15mm, the method was able to accurately judge for crack defect. Also, the measuring accuracy can be improved by 6.4% compared to that of traditional detection method.