With regard to the problem of target threat assessment for air combat. In the improved TOPSIS the subjective and objective factors are synthetically taken into account, so the combination weight is gained by combining the subjective weight of threat assessment index gained by IAHP with the objective weight of threat assessment index gained by the maximal deviation. The mathematical model of evaluating target threat for air combat by improved TOPSIS is established. Firstly, a Canonical decision-making matrix is established; secondly inter-zone positive and negative ideal solutions are fixed; thirdly the distances are calculated between every target and positive (negative) ideal solutions. Finally, the correlation between each target and the ideal solution is calculated. An application example is given and the results show that target threat assessment indexes are the same under three different combinations of weight factors and the method is reasonable and effective.