Abstract: Water filling process is the first step after the completion of construction, or maintenance, of a large pipeline water transfer system, and in this process hydraulic transients have a direct impact on the safe operation of the system. Previous studies of water filling transients mainly focused on the transfer facilities such as open channels, tunnels and inverted siphons, but water filling studies on long pressurized pipeline transfer systems were relatively lacking. This paper discusses numerical models of the water filling transients in a pipeline system, and analyzes calculation methods of the water column separation and rejoining during the filling process. A review on the- state-of-art of numerical models and solution algorithms for simulating air valve operation to protect against water hammer during the filling process has been undertaken, and the direction of future studies on of the water filling transients in pipeline systems is suggested.