Considering the weight value of safety factors in different intervals, a new evaluation method for dynamic safety factor is proposed. Vector analysis method of the block theory is adopted for programming. The detailed research approach is resented as follows: first of all, the required information of structural planes and free face is input to form the unlocated block, and the finiteness and mobility of the block are determined subsequently according to finiteness theorem and mobility theorem. If the block is movable, find out the sliding mode (separating, single face sliding or double face sliding) of the block under the combined action of dead eight and seismic load as well as the safety factor under that sliding mode. Finally the time  history curve of dynamic safety factor is obtained by cycling all the earthquake times. This study can be regarded as an exploration in the evaluation index of dynamic safety factor.