Abstract: Because the development of gas reservoir with low permeability in one offshore sag is still in the stage of practical exploration at present, the micro-pore structure, micro-flow characteristics have not been systematically studied. Based on the experimental data from constant rate mercury injection, nuclear magnetic resonance image, core displacement test, the authors studied the micropore structure characteristics, movable water saturation and the corresponding mechanism of water generated and the law of gaswater seepage flow, and established a chart for the classification of low permeability reservoir by taking into consideration of five characteristic parameters i.e. permeability, porosity, dominant throat radius, gas saturation, movable water saturation. This provides a useful reference for the efficient development of offshore and for the selection of well and layer.