Threat assessment to the target of ballistic missile is an important part in anti-missile defense,which can provide scientific references for missile defense decision-making.To assess threat degree of the targets effectively,an index system is constructed and 7 kinds of important evaluation indicators are given.Then each indicator is analyzed in detail.On this basis,a new threat assessment method is proposed based on TOPSIS (Technique for Order Preference by Similarity to an Ideal Solution) and grey correlation degree,which makes full use of the advantages of two methods.Finally,it is simulated and verified in the typical scenarios.The results show that the threat evaluation method can give comprehensive assessment result of target threat degree,which has certain application value in anti-ballistic missile defense system.