An ultrasonic flaw detection standard test block with a high-precision flat-bottom hole belongs to the technical field of ultrasonic flaw detection equipment and is used for improving the machining precision of the flat-bottom hole of the ultrasonic flaw detection standard test block. The technical scheme is as follows: the flat-bottom hole of the standard test block comprises a chambering sectionand a standard hole section; the reaming section is concentrically connected with the standard hole section; the reaming section and the standard hole section are connected and then are perpendicularto the detection plane of the standard test block; the upper end of the reaming section is open on the plane of the standard test block, the center of the upper end of the reaming section coincides with the center of the flat-bottomed hole of the standard test block, the lower end of the reaming section is connected with the upper end of the standard hole section, the lower end face of the standard hole section is the bottom face of the flat-bottomed hole, and the diameter of the reaming section is larger than that of the standard hole section. According to the utility model, an operator canidentify flat-bottomed hole signals, and flaw detection equipment can be calibrated conveniently; meanwhile, the machining difficulty of the flat-bottom hole with the large length is greatly lowered,the precision of the flat-bottom hole is remarkably improved, and the flaw detection accuracy can be guaranteed when flaw detection equipment or instruments are calibrated.