Abstract: Ordinary thickness tester cannot accurately measure the thickness of cast iron because of the particularity of cast iron materials. From the experiment on cast iron step block used by ultrasonic detector with a A-scanning signal. It is known that the sound velocity, hardness and metallographic structure of cast iron remain unchanged in a certain range of pressure and temperature. Accordingly, we can calibrate the speed of ultrasonic wave in cast irons by flange parts, and understand the advantages and disadvantages of the probes with different frequencies and crystal sizes through comparison experiments of different ultrasonic probes, which is helpful to improve the accuracy of measuring thickness of cast iron dryer.