Experimental results of the basic mechanical properties and failure mode performed on rock salt demonstrate that rock salt is a typical and peculiar soft rock , whose" Young' s modulus/strength(E/S)"ratio is extremely small,and whose failure follows the deformation failure criterion. With these results,this paper develops a theoretic breakdown criterion of hydraulic fracturing in rock salt,which is based on the greatest tension strain failure theory and includes the vertical stress effects. It has been suggested that the critical tension strain should be determined from uniaxial compression tests. This theoretic criterion exceedingly agrees with experimental results and is applicable for every soft rock with small E/S ratio under various stresses.