Earthquake effects are commonly considered in the stability analysis of rock slopes and other earth structures. The conventional approach is often based on the limit equilibrium method by Mohr-Coulomb or Hoek-Brown failure criterion. However, experimental data shows the rock failure doesn’t keep to simplex criterion and it is influenced by the intermediate principal stress distinctly. A new method for evaluating seismic safety factor and permanent displacement of rock slopes is proposed according to the characteristic of joint rock. The unified strength theory is introduced into the limit equilibrium method. In this method, the strength difference and tensile failure can be considered. The influences of angle of bedding slope, inclination of joint, unified strength parameter, tensile depth and seismic load are investigated. The results reveal that, with the increase of intermediate principal stress effect, the safety factor also increases, but the permanent displacement is descended obviously. In practical engineering, the potential strength of filling materials should be sufficiently developed under the guidance of the unified strength theory.