Abstract: We present a method of synthesizing gas pressure based on acoustic velocity at three-frequency point with high accuracy and simple measurement. Firstly, acoustic velocity at three frequency points is measured. Then, relaxation frequency of acoustic velocity dispersion is calculated. Finally, gas pressure is obtained according to the linear proportional relation between relaxational frequency and gas pressure. Simulation results verify feasibility of the algorithm. The method is simple and of high precision for ultrasonic online real-time pressure detection of gas chambers.