To investigate the relaxation characteristic of rock mass discontinuity with different slope ratios and normal stresses, the stress relaxation tests of dentate discontinuity poured by cement mortar on the condition of shear stress were carried out by using biaxial creep machine. The test results show that: the relaxation curves can be divided into three stages, i.e. the instantaneous relaxation stage, attenuation relaxation stage and stable relaxation stage; the shear nonlinear Maxwell relaxation equation is obtained by constructing the relation between viscosity coefficient and time, the curves of the empirical equation agree with the test ones and get change law of initial viscosity coefficient and change rate with shear stress; according to change law of relaxation stress with shear stress and mechanism of determining long-term strength using transition creep law, a stress relaxation method is proposed to determine the long-term strength, which is relaxation stress peak method.