Studies have shown that the gas-water interface maybe relative high in the reservoirs with low permeability and exhibit the undulating distribution due to the lateral heterogeneity of reservoirs. As to the offshore gas reservoirs,it is hard to accurately describe the distribution of fluid by the conventional methods because the capillary pressure data available are limited. By adopting FZI, the article provides a new way to predict the distribution of fluid based on the reservoir physical property. It also makes a close study on the spatial distribution characteristics of iso-saturation interface. Finally, the study achievement was applied in a dynamic analysis referring to a gas reservoir in some sea area of China,and proved to be effective.