In order to assess threat of reentry-course ballistic missile comprehensively and rationally,a concept of defense object is put forward,which is integration of conservation-shot resource and theater critical area;then index system of threat assessment of reentry-course ballistic missile and frame of model is constructed;on this basis,quantized method of index system is gived and threat assessment model of multiple defense object and multiple ballistic missile is established;frame and algorithm flow of grey relationship analysis based on entropy is proposed. Practically applications indicate that the model is valid and the algorithm is feasible.