Abstract: The in-situ water injection experiment is a particularly useful technique to determine the water-resisting ability of rock mass.In order to probe the water-resisting ability of lower coal mudstone seam floor，the water injection experiment in field is made to test two mudstone segments with different fragmentation degrees of Yanzhou Mining Area.With the permeability and the mean of water-resisting strength as the evaluation index，the water-resisting ability of the rock mass is comparatively analyzed.Results showed that the fragmented mudstone was stronger than the thick-bedded intact mudstone in the permeability with limited water resisting，the permeability of the rock mutated when the press reached at critical value.The change of permeability made by water pressure was sustained in fragmented mudstone，but it was instantaneous in thick-bedded intact mudstone.The thick-bedded intact mudstone was stronger than the fragmented mudstone in the mean of water-resisting strength.The conclusions will have the guide role in the cognition of lower coal mudstone seam floor in Yanzhou Mining Area，and the results make an important reference for safety-mining of lower coal mining.

Key words: Lower coal group, Water injection experiment, Permeability coefficient, Mean strength of water-resisting