Abstract: Based on the features of severe slug flow distributed in 4 stages, this paper sets up a one-dimentional quasiequilibrium simplified calculation model for the unsteady flow in the L-type riser system, which ignores the effects of friction and acceleration, and uses the drift flow model to simulate the gas phase penetration. Characteristic method is used to solve the gas phase penetration stage and gas liquid eruption stage. The gas density offset correction is introduced to simulate the typical severe slug flow and transitional severe slug flow in which the length of the liquid slug is smaller than the height of the riser. Comparison of calculated results from the model with the laboratory results and experimental data from other literatures shows that the built model is able to make a rather accurate simulation and prediction on the two-phase unsteady flow in the L-type riser system.