The measure of sound speed is an important part of general physics experiment at the university. The standing wave method is widely used in the velocity measurement experiment. Sound speed is measured according to the character that, the composed wave state between incident wave and reflected wave is changed cyclically as the distance between reflecting interface and sound source changed. However, the practical situation is much more complicated than the ideal situation. In the experiment of measuring sound speed by standing wave method,the resonance curve of ultrasonic wave was obtained through pointwise measurement by using digital oscilloscope, and some anharmonic false peaks were observed. It is suggest that we should judge the size of the maximum value of the amplitude, wipe out the false peak, to minimize the measurement error.