Abstract: With the development of tight gas, traditional capillary?equilibrium theory that initial water saturation is almost equal to irreducible water saturation is questioned. Taking the typical tight gas reservoirs in northern Ordos basin and western Sichuan basin for examples, their well logs, sealed coring and core lab analyses indicate that the initial water saturation is lower than the irreducible water saturation,namely, there exists ultra?low water saturation for such a reservoir. The reservoirs with low initial water saturation have high spontaneous imbibition potential, leading to the increase of water saturation nearby the wellbore and fractures, and the decrease of gas permeability, which is water phase trapping damage. The analysis of well logging data and the gas transfer experiments show that water phase trapping damage could induce fluid sensitivity damage and aggravate stress sensitivity damage. Water phase trapping impairs gas permeability nearby the wellbore and fractures, prevents from the gas production, obtains low gas saturation from well logging, no gas rate during the well test, and causes to undervalue such a gas reservoir. It is crucial for effective development of tight gas with abnormal low water saturation nearby the wellbore and fractures to prevent from capillary imbibition and aqueous retention