Abstract: In order to study and analyze water affected to the physical mechanics performances of the muddy siltstone,based on the selection on the muddy siltstone with three moisture contents of dry,natural and saturated moisture as the study object,an uniaxial compression failure and scanning electron microscope experiment were C onducted and a study was conducted on the water affected to the mechanics features of the rock fracture,failure form and micro-structure pattern.The study results showed that water bearing could make different changes in the stress-strain curves of the muddy siltstone,the sustained time of the elastic stage would be reduced and the plastic yield stage would be obvious steadily. With the moisture content increased,the peak stress and elastic modulus of the muddy siltstone would be reduced in index.The water bearing could make softening of the muddy siltstone ,the failure serious degree would be reduced,the energy released process would be mild and the muddy siltstone would be changed from fragility or semi-fragility to plasticity. The water bearing could change the micro-structure of the muddy siltstone ,the particle would be swelling with water,t he arrangement would be in disorder,the structure would be relaxed,the cementing material would be dissolved and the cohesion would be reduced.The micro-structure Wo uld have obvious influences to the macro mechanics performances of the muddy siltstone.