A 2D CFD model for the full size experimental facility was developed, and the severe slugging phenomena under different conditions were numerically simulated to study the transient characteristics of severe slugging by using Fluent. In the simulations, the VOF method was adopted to track the gas-liquid interface, and the long subsea pipeline was equivalent as a tank to improve calculating efficiency. Four flow patterns were numerically simulated including typical severe slugging (SSI), severe slugging II (SSII), transition to severe slugging (SST) and stable flow. The simulation results of the four flow patterns were compared with the experimental data. Finally, the transient characteristics of the typical severe slugging were analyzed thoroughly. The results show that the present numerical simulation methods can be used to simulate the severe slugging phenomena accurately and these methods are reasonable for the simulation of severe slugging in long riser system. Under the present condition, the maximum instantaneous mass flow rate can reach the 2.5 times of the inlet, and the instantaneous average flow velocity can reach 4.5 times of the inlet liquid velocity. These analyses have important implications for the hazard assessment of severe slugging.