Abstract: The original interface and seepage channel of water rock interaction is provided by original and mining crack of rock in coal measure strata.Due to the mineral composition and cementation type and other factors, the mechanical properties of the rock in coal measure strata is changed significantly under the water action.The mineral compositions of pelitic siltstone, the moisture content of pelitic siltstone changes with soaking time and the evolution characteristics of strength damage of pelitic siltstone under the impact of moisture content were discussed by using X-ray diffractometer, nondestructive immersion test equipment, mechanical properties and acoustic emission monitoring system.The results show that kaolinite, illite and chlorite constitute 62% of the mineral composition in pelitic siltstone.Rock samples are affected by water.With the change of time, the moisture content of rock samples is divided into three stages, including swift growth (0-25 h), slow growth (25-85 h) and constant stage (85 h-).From dry to saturated, the uniaxial compression strength of the rock sample is reduced by 47.72%, and the modulus of elasticity is reduced by 18.36%.With the increase of moisture content, the destructional forms of rock samples is gradually transformed from shear failure to tensile failure.Change relationship between stress and accumulated AE counts and time can elucidate the stages of crack development.Among these stages, acoustic emission signals of crack instability expansion stage and post peak are active.Except of rock whose moisture content is 1.6%, the result of acoustic emission localization is roughly consistent with the actual failure trace of rock samples.At the early stage of loading, shear cracks which are located inside the core appear and expanded.Crack and energy spread to the periphery.At the later stage of loading, tensile-shear composite cracks occur.The research findings provide useful references to water rock interaction problem in engineering such as water conservation mining in coal mine, underground reservoir and water-rich roadway.