Abstract: According to the small perturbation theory and solvable conditions of one-order linear equations,the dispersion equation of two-phase pressure wave propagation with phase change was derived via the ensemble average two-fluid model.The relationship between critical flow velocity and propagation speed of pressure wave and pressure pulse was analyzed,and the propagation characteristics of pressure wave in the bubbly and slug flow were studied.The results indicate that the pressure wave in gas-liquid two-phase flow has the dispersion property,and the propagation speed of pressure wave with indefinite angular frequency is the propagation speed of the pressure pulse.Moreover,the two-phase critical flow velocity is equal to the propagation speed of the pressure pulse.