To make suggestions on the pipeline safety design and apply the transient-based technique into the fault detection in the viscoelastic pipeline, this paper studies the gas-liquid two-phase transient flow in the viscoelastic pipeline. Firstly, the fast and complete valve closure is used to produce the gas-liquid two-phase transient flow in the gravity flow pipeline made of plexiglas. Secondly, by using the air fraction to modify the transient wave speed, two one-dimensional gas-liquid two-phase transient flow models, which take into account the influence of unsteady friction and pipe-wall viscoelasticity, are established based on the discrete vapor cavity model (DVCM) and discrete gas cavity model (DGCM). Experimental and model results show that DVCM can accurately simulate the experimental wave speed in the low pressure system when the initial flow regime is bubbly flow, while the average wave speed obtained by DGCM is larger than the experimental wave speed. In the simulation of transient bubbly flow, the results of DVCM are in good agreement with the experimental values, while the maximum peak values in the results of DGCM are larger than the experimental values, which demonstrates DGCM is more suitable for the design of pipeline safety. During the gas-liquid two-phase transient process, the compressibility of air makes the effect of pipe-wall viscoelasticity on pressure damping greatly weakened, which leads to the unignored effect of unsteady friction, and the pressure damping in the whole transient process becomes slow due to the presence of air.