Abstract: This paper introduces the work theory of NDT ultrasonic flaw detection instruments and details all kinds of preparing items: manufacture of sample rail, sensibility adjustment of ultrasonic instruments and exacted repetitive test of standard sample rail. This paper makes a lot of application experiments of NDT in the high speed rail ultrasonic flaw detection. Following conclusions were drawn through stating, contrasting, assaying and analyzing: the whole capability of NDT is stably and credible; and the inspection rate of high speed rail is lower than other rails. Although the phenomenon of undetected fault during inspection is not found, there are still a few miscalculations, many of which are centralized in the middle of checked rail due to oxidation, sulphuration and silication.