Abstract: Its internal micro structural characteristics and the physical and mechanical parameters will change when rocks are subjected to water.These changes are mostly associated with the inner structures and components of rock mass.The floor limestone of North China permo-carboniferous type has a typical feature that it is under double action of mine pressure and high pressure water.In this case，under the special stress condition，the strength and deformation characteristics are more complex than the common rock.It shows that mineral compositions and micro structural characteristics，such as pore，fracture and other physical parameters，will change greatly under water immersion environment.Such differences will ultimately affect the strength of rock.Based on these，components identification and microstructure are analyzed by means of advanced instruments in the lab，and the effects of water on the properties are qualitatively analyzed.Meanwhile，grouting quality and water softening tendency are evaluated.The researches on ingredients and micro structure characteristics in different moisture content for Benxi limestone have important guiding value for a reasonable explanation of their mechanical properties and its engineering properties.

Key words: Floor rock, Benxi limestone, Hydro-physical property, Components identification, Micro-structural characteristics, Grouting reinforcement, Softening mechanism