Water phase trapping is the most vital damage to tight sandstone reservoirs, which is different from water blocking, and it can prevent tight gas reservoirs from exploration success and economic exploitation. It cannot be evaluated by the traditional method evaluating water blocking. This paper takes example from Daniudi tight gas reservoir in Ordos orthern Basin, China. The potential tendency to water phase trapping damage is strong by the research to the geology characters such as abnormal low water saturation, pore structure. The severity of water trapping in Daniudi gas filed was proved by the experiments of the effects of water saturation to gas relative permeability, spontaneous imbibition and phase retention of water and dill-in fluids. Based on the influence mechanisms of initial water saturation, reservoir pressure, liquid properties and pore structure on water phase trapping, phase trapping coefficient (PTC) was put forward to predict the potential of water phase trapping damage, and it is dimensionless, and is characteristic of comprehensive consideration of the factors, convenient and widespread use. The saturation and invasion depth of filtrate varying with time around wellbore were simulated, the results show that if no filtrate cake on the well wall the Daniudi gas reservoirs, especially P1×2+3 gas reservoir is prone to be damaged by water trapping to be wrongly evaluated or to be undervalued. In view of the theories and research results, serial measurements to prevent and treat water phase trapping are adopted to protect gas reservoirs and P1×2+3 gas reservoir was detected to become the major pay formation. The practices of gas formation damage control verified the results of evaluation.