Conventional air valve models usually fix the air pocket at the air valve node and neglect boundary movement caused by pocket volume variation during air intake and discharge, which may lead to pressure calculation errors at adjacent nodes. To overcome this limitation, an improved air valve model, namely the spreading air valve model, is proposed. This model updates the air-pocket boundary position in real time according to the variation in air-pocket volume, so that the motion of the water-air interface is more consistent with the actual physical process. By coupling the gas state equation with the fluid governing equations, the dynamic and simultaneous solution of air-pocket pressure, volume and mass is realized. Validation results from a pressurized water pipeline project show that the spreading air valve model can effectively simulate the hydraulic transient process, reasonably predict water column separation and water hammer pressure variation, and significantly improve the accuracy and reliability of water hammer protection design for pipeline systems.