Abstract: The mudstone of Mianxian-Ningqiang area in the southern Shaanxi is mainly composed of clay minerals,which characteristics of high shale content,loose rock structure,weak anti-weathering ability and poor physical and mechanical properties.Especially contact with water the mudstone is rapidly expansion,disintegration and softening in the short time,and is significantly for rock fractures development and rock strength decreased,which is dad for Mian-Ning freeway construction.Based on this analysis,water immersion test of mudstone and water interaction is designed and the following conclusions are obtained in this paper: the mudstone failure is easily occurred in the position of montmorillonite and other unstable mineral and fractures are almost development along the horizontal bedding after soaking.With longer soaking time,the rupture surface is formed as fracture transfixion when fractures developed to certain degree.In the circulatory state of saturated-weathering - saturated,the rock fractures are more development,the mechanical properties are more decreased,the rate of disintegration and softening are more fast and the rock failure are more easy than in the state of always soaking.