Based on the consistence principle for the severe slugging formation condition, a computational fluid dynamics (CFD) method is proposed for numerically simulating the gas-liquid severe slugging in a pipeline-riser system by converting the three-dimensional pipeline-riser system into a two-dimensional equivalent one. Numerical simulation is conducted for the gas-liquid flow patterns of the severe slugging in a declination pipeline-riser system according to the experimental cases presented in the reference, and variations of characteristics of the flow parameters with gas-liquid superficial velocity due to such a severe slugging are obtained, including period, pressure fluctuation and blowout time, the numerical results are in good agreement with the experimental results. Moreover, the theoretical methods are obtained of determining the gas superficial velocity at the riser inlet, the gas volume fraction in the riser and the average velocity at the riser outlet during the severe slugging blowout stage, and the variations of characteristics of these flow parameters with time are presented. Besides, a theoretical predicting method of determining the flow patterns in the riser during the severe slugging blowout stage is further proposed, and the theoretical results are consistent with the CFD numerical results. The CFD method could save much time and computing resource, and the theoretical methods could be used to predict the damage of severe slugging quickly.