Abstract: The chlorite schist sample from the Jingping Ⅱ Hhydropower Prjoect on Yalong River was saturated by the saturated NaCl solution and the distilled water, its uniaxial and conventional triaxial test was done in the RMT and MTS test machine, in which the strength index of saturated and dry state in different solutions was obtained. Compared with its mechanical parameters' change and analyzed its micro\|mechanism of electric double layer, results showed that the strength of chlorite schist under water was decreased obviously, which showed its softening under water because of the influence of its electric double layer, the more the thickness of electric double layer was, the more the decrease of the chlorite schist's strength was, vice versa.