By building a schlieren optical system, the visualization of a 40 KHz sound wave is realized. By using the schlieren system, the demonstration of acoustic reflection, interference and diffraction can be easily realized. At the same time, the visual acoustic field provides another simple way for the measurement of sound velocity. In this paper, the experimental device is simple in structure and low in cost. The experimental content combines the optical and acoustic content, which can be used as the acoustic experimental content for undergraduate students.