Abstract: In the present paper, a numerical method for simulatingthe characteristics of two-phase flows in a declination pipe-riser systemwith the gas and fluid was developed. The Brackbill modelwas applied to simulate surface tension between two phases and the VOF method was used to capture themoving interfaces between gas-liquid phases. At low superfical velocity of gas and liquidsuperficial velocities, severe slug flows in such a pipe-riser system weresimulated and the variety characteristics for flow parameters were analyzed.The results show that the flow parameter characteristics due to such asevere slug flow have remarkably periodic characteristics, including theflow pattern, pressure, slug velocity, average phase velocity at the riseroutlet and gas volume fraction, and severe slug flow in a period consists offour evolvement stages where the characteristics of such flow parameters ineach evolvement stage were further given. The numerical results are goodagreements with experimental results reported in the references, showingthat the proposed method is effective.