Abstract：The mechanical behaviors of sandstone with large porosity are different from the other rocks；the research of its mechanical properties is very important not only in the theory but also in the practice. The two sorts failure theories are used to explain the failure types of sandstone with large porosity based on the laboratory research under different saturated liquid conditions；then the conventional Cap Model is extended and is used to simulate the testing results. A critical value of confining pressure is proved to exist during the process from mainly pressure-shear failure to mainly slough failure. When the confining pressure is increased，the mainly pressure-shear failure can be changed into slough failure. A critical saturated degree also exists during the change processes from oil saturation to water saturation. When water saturated degree is above critical value，the mechanical features of sandstone with large porosity are changed clearly. The theoretical and testing results explain reasonably the subsidence from oil exploiting by water injection.
