The "hypotaxis stability" concept and the dynamic stress hypotaxis safety ratio method were presented in order to take the safety of arch dam into consideration because the safety coefficient does not assure the safety of dam.A three dimension dynamical contact unit model was developed to illustrate fractured rock mass of the dam abutment slope.Using the 3D-dynamical constraint joint element,certain dam abutment slope system was analyzed.Numerical analyses show that: with dynamic stress hypotaxis safety ratio method and the three dimension dynamical contact unit model,the "hypotaxis safety ratio" of abutment slope can be decreased by 20%.