Abstract: After analysis of the cause of formation of cracks on outer slope of the Sanyuan Reservoir dam,reduction and dedution of physiscal parameters by using rock and soil mechanics theory and method,the author suggests that the cracks were formed by creep sliding deformation of local shallow dam-body earth under the influence of water instead of deep dam-body-earth sliding.Hence a series of corresponding crack treatment measures have been susggested for consolidating the cracks and stablizing the dam.