Offtake (gas recovery per year) and production plateau duration(period of stabilized production) are the two important development indexes. This paper supplied one method to calculate the plateau duration corresponding to offtake of a constant volume gas reservoir by combining deliverability equation and material balance equation. The result was proved to be reliable by reservoir simulation, which means that the method by this paper is practical. In addition, this paper proved theoretically that offtake is in linear relationship with (1-Rpsp) 2 (Rpsp is the recovery percent at end of production plateau).