Abstract: Water vapour in gas condensate reservoir has effect on the phase state of the gas condensate system. On the basis of phase state experimental analysis on the gas condensate system with water vapour, we analyze how water vapour has effect on the phase state of the gas-condensate system and summarize the phase behavior of the gas-condensate system with water vapour. If water vapour occurs in the gas- condensate system, the pressure-temperature figure area increases, the retrograde condensate area increases and initial retrograde condensate pressure increases in the phase diagram; serious polarity of the water vapour will accelerates condensate oil volatilization; the retrograde condensate-pressure figure of the gas- condensate system with water vapour and without water vapour will across each other in the constant composition expansion diagram and the constant volume depletion diagram. The phase state of the gas- condensate system with water vapour is different from that without water. Gas deviation factor of the gas- condensate system with water vapour is smaller than that without water vapour at the same pressure and temperature. Gas recovery coefficient, condensate oil recovery factor, heavy component content in well fluid and flash vapour of the gas-condensate system with water vapour is bigger than those of the gas- condensate system without water vapour.