Abstract: A transient hydrodynamic model of two-phase flow is established according to two-fluid model. The system of governing equations is proved to be well posed by means of characteristic analysis. With eigenvalues and left-eigenvectors derived from the model equations, the partial differential equations are transformed into ordinary differential equations. The characteristic line scheme based on preliminary estimate and correction difference is adopted for numerical simulation and corresponding programs are made. Experiments of rapid increased and decrease of flow rate of gas phases are conducted on large multiphase test loop of 80 mm ID and 380 m in length. Comparisons of calculation and experiments demonstrate that the scheme achieves good convergence and stability and the programs make good prediction of pressure and void fraction in gas flow rate transients and are capable of analyzing various parameters in transient flow.