Abstract: The principle of ultrasonic time of flight diffraction(TOFD) method was analyzed. Based on ultrasonic A-Scan, the TOFD imaging principle was investigated. Two scan methods of TOFD, as parallel and non-parallel scan methods were discussed, and the second was analyzed in detail. The calculation formulas of up and down refraction angle were given, and then the scan range was analyzed. The TOFD imagines of work piece contained defect or not were given, the TOFD imaging principle was investigated more deeply, and the character of TOFD testing was concluded. Finally, the dead area of TOFD testing was investigated, the calculation formulas of up and down dead area were given, and according to the standard CEN/TS 14751—2004, the dead area of five different thickness work piece was given.