Abstract: Due to the special hot rolling process, the respective sound wave velocity and attenuation in the rolling and non rolling directions of TMCP steel is quite different. Compared with ordinary steel, ultrasonic testing of TMCP steel weld needs more complex processes. The calibration accuracy of the ultrasonic instrument plays an important role for locating defects and quantitatively evaluating defects. Starting from the calibration block of TMCP test and carrying on the calibration of velocity and other parameters by the combined use of contrast test block and standard test block of type ⅡW, corresponding DAC curves were made. Finally, the accuracy of the instrument calibration was verified through the test of artificial defects. The verification results show that the calibration method can perform accurate and reliable localization and quantification of defects.