A pump−pipeline experiment platform is designed and built for exploring the dynamic flows in a pipe-conveying fluid with entrapped air, and a corresponding CFD simulation is executed based on the volume-of-fluid (VOF) model. The transient flow characteristics of the entrapped air in pipelines under different pump and valve working conditions are studied by experimental and numerical approaches. The experimental data and simulation results are in good agreement. Both of them show that the pressure impact can be mitigated by increasing the air volume ratio (air volume/pipe volume) or the valve opening time. Two types of the air-water interfaces, a clear interface and a broken interface, appear in the process of air pressurization to the peak pressure as the flow rate rises and the valve opening time decreases. The influence of the flow rate, air volume and valve opening time on the peak pressure and arrival time are investigated, and a corresponding equation with respect to the dimensionless parameters is proposed for the pressure prediction of a system with entrapped air during start-up.