Abstract: Based on the non?stabilized well test data in the early stage of water drive gas reservoir without water production, combined with the present production performance, this paper determines the current gas permeability by means of gas binomial deliverability equation of gas?water two?phase stationary flow, by which gives the current water saturation in terms of gas?water relative permeability curves in this reservoir, and then using material balance principle of water influx, establishes the relationship between the water influx and the dynamic reserves. Integrating this relationship with the material balance equation of water drive gas reservoir, the dynamic reserves and the current water influx of such a reservoir can be determined finally. The case study indicates that this new method is accurate and practical by avoiding complicated calculations of water influx