Abstract: For ultrasonic time of flight diffraction (TOFD) method, longitudinal wave sound field of probe cannot cover the entire space of the inspected weld, and there is a large range of insensitive area near the surface of the inspected weld. An improved ultrasonic TOFD method was proposed, namely defect detection method of insensitive area. In this method, ultrasonic longitudinal wave reflection on the bottom surface of the inspected specimen was employed to make ultrasonic sound field cover the near surface area, and this method was adopted to detect artificial defects and welding defects. The results showed that the mean error of buried depth of artificial defects was 0.3 mm, and the mean errors of buried depth and transverse position detection of lack fusion defect in the weld were 0.6 mm and 0.4 mm, respectively. The proposed defect detection method of insensitive area can effectively detect near surface defect, and it is an extension and supplement for the conventional TOFD method.