In order to study the disintegration process and micro mechanism of mudstone under water action,the paper used X ray diffraction and laser scanning confocal microscopy to measure the composition of mudstone and the change of three-dimensional topography after water saturated，announced the change law of mudstone topography and analyzed the micro mechanism of mudstone disintegration.The results show that the quantity and width of crack increase with the increase of saturation time. The disintegration of mudstone is caused by the second porosity and wedging pressure in saturation process. The whole second porosity is caused by the second porosity results from chemical effect of water and the action of water flushing and transport. Crack is formed by wedging pressure of porosity surface in the process of saturation. The sample is disintegrated because of crack penetration each other.