Triaxial compression experiments of slates under different saturated conditions were done through the XTR01 electric- fluid serving compression machine

which were taken from the engineering of the western route of Water Transfer from South to North China. Relative saturation was defined as the ratio of mass of natural saturated rock to complete saturated rock; and the saturation was used to analyze the change of mechanical parameters （rock strength

elastic modulus and Poisson’s ratio）. And an attempt was made to explain the change mechanism by the micro-mechanics model. With all the efforts

some conclusions are drawn as follows: 1)the triaxial compression strength of slates fell down when saturated in water and the most descendent can be 46%; and this change can be described by a law with the relative saturation; 2)when saturated

the elastic modulus and the Poisson’s ratio increased with the accretion of the relative saturation

but the law was not too clear for the anisotropy of the slates; 3)theoretical analysis has proved that the saturated rocks in geology conditions will undergo two process as drained and undrained

when the saturation increased

the elastic modulus and the Poisson’s ratio will increase simultaneously

which has confirmed the experiment results; 4)the micro-mechanical model shows that

when saturated in water

the absorbtion of the slate surface and the decrease of the capillarity among the grain are the main reasons to the water-weakening effect appeared in the slates.