Abstract: Abstract: In this paper, we improved an existed two-fluid model for simulating the transient flow with liquid-column separation in the pipeline system. An improved NND numerical algorithm was proposed to ensure the accuracy and stability of the model. The finite relaxation and the infinite relaxation methods were compared. It was concluded that when using the finite relaxation calculation method, the relaxation coefficient was too large, resulting in the reverse correction for vapor-liquid phase pressure difference in the equation, which would cause the interruption of calculation. When the calculation method of the infinite relaxation coefficient was used, a stable calculation could be performed without setting the initial concentration of the foam particles, whilst the transient phenomenon could be described more accurately with increasing intensity. In the meantime, due to the forced correction of the void fraction and internal energy distribution after each time step, the method would often produce a larger void fraction and void fraction residue. The water hammer transient tests in two different laboratories were compared to verify the accuracy and applicability of the improved gas-liquid two-phase two-fluid model. We found that for the calculation using the finite relaxation parameters, the phase changes due to cavitation needed to be considered, and also needed to select reasonable bubble concentration to ensure the accuracy of calculation. For the calculation using infinite relaxation parameters, the cavitation model can be neglected, but the model could be used to directly analyze the phenomenon of liquid column separation caused by water hammer phenomenon. As the infinite pressure relaxation calculation method was a kind of mandatory correction calculation method, after each time step, it was considered that the gas phase and the liquid phase pressure reached the equilibrium state, and in fact, the relaxation time of the pressure was usually 1 ms. If the calculation time step was less than 1 ms, due to the forced correction of the calculation method, the two-phase flow in the unbalanced state was forced to be in equilibrium, which led to the correction of the void volume became too large, making reduction of the cavitation model in the low pressure area. The use of improved two-fluid gas-liquid two-phase flow calculation model with a reasonable numerical calculation method and infinite relaxation parameters can be used to predict the transient phenomenon of gas-liquid two-phase pipelines with obvious liquid column separation. The method had high calculation accuracy and it was suitable for water hammer protection in industrial pipelines calculation with a better value of application.