Abstract: As a result of the study of unsteady-state flow, a practical calculation method and formula of the characteristics method were derived from the equations of mass conservation, momentum conservation, energy conservation that were established according to the basic laws of hydrodynamics. The partial differential e-quations set difficult to solve was transformed to the equivalent difference equations set convenient for numerical calculation. Therefore, the calculation of quick transient flow in gas pipeline is accomplished. The method has explicit conception, rigorous mathematical analysis and high precision.