Considering that it is difficult to make threat assessment in anti-TBM (Tactical Ballistic Missile) combat with incomplete index information by using the existing threat assessment methods, a threat assessment method based on Evidential Reasoning (ER) was proposed.First, the threat assessment factors of TBM in boost phase were analyzed, the hierarchical structure model for threat assessment was constructed, and analysis was made to the issue of incomplete index information.Then, the ER method was applied to conduct threat assessment in anti-TBM combat with incomplete index information, and the detailed steps of the method were put forward.At last, analysis of examples showed that the proposed method can deal with the threat assessment problem in anti-TBM combat effectively.