Based on the acousto-optic effect between the optical frequency comb and the ultrasonic pulse, we propose a method for direct measurement of seawater sound velocity. We measure the flight distance of the ultrasonic pulse according to the phase of the beat frequency between the optical frequency combs, and measure the flight time of the ultrasonic pulse based on the intensity variation of the zero-order light caused by the pulse acousto-optic effect. A measuring device based on Mach-Zehnder interferometer is built. The experimental results show that this method can achieve high precision seawater sound velocity measurement in lab, and the measurement uncertainty is better than 5 cm/s.