Ultrasonic grating is design and built.Clear diffraction spots are obtained while a laser beam passes through. By measuring the distance between spots，the velocity of sound iscalculated and analyzed. Furthermore，the velocity of sound in pure water with different temperatures and Nacl solution with different concentrations are measured. Dependence of the sound velocity on the temperature and liquid concentration is established. T velocity of sound increases linearly with temperature and liquid concentration. The velocity increases 2.09m/s for 3.974MHz wave，and 2.04m/s for 16.574MHz wave when the temperature increases for 1℃. The velocity of sound increases 13.6m/s for 3.974MHz wave，and 11.8 m/s for 16.574MHz wave as the concentration of NaCl solution increases 1%. The difference in velocity of sound with different frequency is ascribed to inaccuracy of measurement.