The invention discloses an ultrasonic time of flight diffraction (TOFD)-based offline defect judgment method, which is characterized by comprising the following steps of: 1, scanning a body to be detected by using an ultrasonic detector to acquire lateral waves, bottom echoes and A scanning signals of the body to be detected, and synthesizing to obtain a B/D scanned picture; 2, accurately positioning the lateral waves and the bottom echoes by using an image processing and lateral peak search combined method to complete computer calibration of an ultrasonic atlas so as to obtain a plate thickness value of the body to be detected; 3, straightening and eliminating lateral waves with overlapped near surface defects by using peak calibration and a difference algorithm; and 4, performing wave beam sharpening and image enhancement treatment on a B scanned picture by using a synthetic aperture convergence technology to obtain an accurate position of a weld defect. By the method, near surface and internal defects can be effectively identified and accurately positioned, and the method can be widely applied to the field of ultrasonic nondestructive detection.