Assessing the threat of TBM timely and exactly is needed in making plan and working out the strategy about Ballistic Missile Defence system. According to the offence-defence power and the weapon characteristics of both the offence and defence sides, the TOPSIS of the multi-attribute decision-making theory is adopted in this paper. And the algorithm of the Tactics Ballistic Missile threat level assessment is provided and the mathematic model of the missile threat assessment is established. The algorithm is capable of sequencing in multi-attribute inputting and multiple decision-making optimising and of adjudging timely, so it is adapted to transient missile offence-defence state. Additionally, the feasibility of the algorithm is verified by the simulation example.