For volumetric closed gas reservoirs

when the gas well produces with stable production rate

it can be divided into unsteady state stage

transient stage and pseudo steady state stage according to the variation behaviours of flowing bottom hole pressure and producing time. The unsteady state stage is called the elastic one-phase stage

and the pseudo steady state stage is called the elastic two-phase stage. The latter is a dynamic method to evaluate the Original Gas in Place（OGIP） controlled by gas wells. The elastic two-phase method proposed by CHEN Yuanqian in 1991 in the pseudo steady state stage has been repeatedly listed in the National Oil and Gas Industry Standards

and has been widely valued and applied in China. Based on the diffusion equation considering the elastic expansions of natural gas

rock and irreducible water in a gas well with stable production in the finite closed gas reservoir boundary

a new elastic two-phase method for the pseudo steady state stage represented by flowing bottom hole pressure and cumulative production rate is derived

which can be used to evaluate the original gas in place（OGIP）controlled by a gas well. The new formula of elastic two-phase method is superior to the old formula which based on the relationship between flowing bottom hole pressure and production time

and the new can overcome the influence of production fluctuation on the evaluation results. The application shows that the new formula of elastic two-phase method is practical and effective.