Abstract: To avoid complex water influx calculations, a new formula was derived to calculate gas well controlled reserves and water influx based on water storage volume coefficient which was expressed by the gap between current water saturation and initial water saturation of the gas reservoir.Using the new formula, the controlled reserves and water influx of three wells in a water driven gas reservoir were calculated.The results showed that this method provided a precision equivalent to other methods.Moreover, the formula was used to calculate water saturation, single well controlled reserves and water influx of a well in such reservoir at different times, which indicated recovery degree, water saturation of gas reservoir increased and gas well controlled reserves and water influx increased over the well production time.The well controlled reserves had a linear relation to formation pressure difference or water saturation, while water influx had a linear relation to the formation pressure difference and an exponential relation to the water saturation.This method requires steady production data to calculate the water saturation, so it is only suitable for the gas wells that have been produced for some time.