In a variety of materials, the ultrasonic propagation velocity is different. Using ultrasonic reflection principle in the material, the measuring instrument detected the thickness of the measured object. AT89C51 microcontroller as the system core, with ultrasonic probe, transmitter circuit, receiver circuit, counter circuit, LCD display circuit together, completed the measurement of materials thickness. The measurement range was up to a 1.2 ~ 200mm. Practice shows that the instrument is low cost, high speed, high precision, reliability, adaptability, etc. So it has broad application prospects.