Abstract: Ultrasonic sensor produced by Banner was used to track a period of broken line for corrugated board at three velocities,and TMS320LF2407A was used to sample the analog signal of the sensor and transform it to digital signal.Through the function of fast Fourier transform,the sample data was analyzed to get the frequency range of target signal.Then a Butterworth digital filter was designed.Filtered data showed that the waveforms at the three velocities were very similar,the signal changed significantly at the junction position between groove and hypotenuse surface of the corrugated board,and the signal were leading or lag at the junction position between the top and hypotenuse surface of the corrugated board because of the characteristics of the scattering and reflection of ultrasound.The former was chosen target position.The broken line from the given point was tracked repeatedly at welding tractor speed of 500 cts/s.The results indicated that the signal mutational site was the broken position,and the error range was -0.60 to -0.04 mm.