Estimating the object threat degree is the key of multi-object attack decision making. In this paper air-combat situation and air combat ability are chosen as the threat indexes. Based on the sum of squares of deviations, an optimal combination weights method is proposed in order to calculate each index's weight, which is contained in many kinds of determining weights methods. Finally, an application example is given, and the result shows that the method is effective in multi-object attack decision- making.