Abstract: Gas deviation factors in formation conditions are the critical data to calculate gas reserves.As to abnormal overpressure gas reservoirs,several methods such as experimental determination,chart and analytic model can be used to determine gas deviation factors.A case study has been carried out in abnormal overpressure gas reservoir in the lower section of the fourth member of the Xujiahe Formation,the Xinchang Gas Field.It has been concluded that in abnormal overpressure reservoirs,gas deviation factors can be determined by means of experimental determination(PVT),chart under high pressure and LXF analytic model.