Abstract: Pu67 gas reservoir features in severe anisotropism,abundant little faults and complicated gas-water relation.Water is taken as reference phase to probe into the reservoir source of producing water.Water fugacity is described by activity and standard fugacity to characterize the strong polarity of water.Material balance model and thermodynamics model of gas-gas-liquid 3-phase equilibrium of condensate gas with water are established in this paper to simulate saturated water content in condensate gas of well Puqi-10 during constant volume depletion.Saturated water content in condensate gas increases with the pressure decreasing.Saturated water content is as much as 1.03 m3/104 m3 when the pressure depletes to 5 MPa.Stable water production per day of well Puqi-10 is 1.7 m3 according to stable gas production per day.The water source of producing horizon can be determined by comparing saturated water content and stable water production per day.The result shows that producing water in S4S horizon is different from the water between the gaps and irreducible water,and there is a little water body in S4X horizon,so producing water mostly comes from formation water.