Schlieren method is an effective method for studying the sound field in transparent medium which called phase objects. The method is used to research the acoustic field by analyzing the refractive index changes induced by the acoustic wave. Calculations of the sound pressure distribution radiated by the line-focus ultrasonic transducer are implemented, and the acoustic field in the focal spot area is obtained. Then the diffraction light intensity on the Fourier transform plane in a Schlieren system is calculated with two-dimensional Fourier transformation when the phase object is the ultrasonic wave. Because the light intensity of the different diffraction spots on the back focal plane of the transform lens depends on the sound pressure, the sound pressure in the focal spot area can be determined using Schlieren technique. A Schlieren system is set up. The images of the line-focus ultrasonic field are obtained and the sound pressure in its focal point area is measured non-invasively by measuring and comparing the light intensities of the different diffraction spots. This work is supported by the National Natural Science Foundation of China (No. 10804085)