Production decline curve analysis is one of our common methods to estimate production rate profile and ultimate recovery of a tight or shale gas reservoir. Because of the long transient behavior of low-permeability gas reservoirs, early production data generally exhibits fracture-dominated flow regimes and late-time flow regimes have not reached yet. These challenges the robustness of production decline curve analysis in such reservoirs, and this also question the validity of several models presented in the literature. If a well produces at a constant flowing pressure, a log-log plot of rate divided byover cumulative gas production vs. cumulative gas production yields a straight line, and cumulative gas production vs. time yields another straight line as well. Based on these two relationships and real data observations, new production-decline models are proposed in this paper. The proposed models are verified by a large number of well production data in tight and shale reservoirs. These models are also validated by numerically simulated cases. Under some special conditions, the proposed models can be used to validate some published models. Many real data and simulate data show that the proposed models can be reliably used to estimate production rates and the ultimate recovery in tight and shale reservoirs.