Abstract: This paper presents that under the filtration pressure the coherent forces between slay particles in the cake consist of two party; one is the Van der Watt force，another is addition-al Coherent force which is relative to filtration pressure. The macroscopic mud cake proper-ties，such as permeability，porosity，thickness.compressibility and diltration pressure，will affect the coherent forces. As a result，it will affect the shear resistance of a mud cake.

Key words: Mud cake, Coherent force, Shear stress, Shear rate