Knowledge reasoning in which knowledge acquisition and reasoning strategy are key techniques, is one of the most important methods applied to threat assessment. Based on the specialty of threat assessment in complex battlefield environment, an information system of threat assessment was built. Then the knowledge in the form of minimized production rules that contained the expert experience was mined by decision logic language. Furthermore, through analysis of knowledge uncertainty, a strategy of knowledge reasoning was raised. In this way, we can estimate the threat of multi-types targets in complex environment, which provide a basis for decision-making of combat command.