With the Shiyaodian Coal Mining Dam as an example, the FEM strength reduction theory combining with numerical simulation analysis is applied. In consideration of the dams mechanical stabilization and dam destruction by stress, the Shiyaodian Coal Mining Dams stress fields and displacement fields are analyzed by the finite element software in two conditions. One is the dam only exerted by the effect of gravity,the other by slags pressure under the condition of designed amount of slags and the safety factors are 2.24，1.39 ?respectively. When the strength parameters of the slope are reduced by 1.39 and the dam is in the state of damage, the maximum stress and displacement fields are obtained, which can provide a reliable evidence for safe operation of the dam.