Abstract: In this paper,first a Bayesian network reasoning model for situation assessment is built according to the causal link between the battlefield situation and events. Then a problem occurred when applying junction tree algorithm to situation assessment is analyzed.Combining with the characteristics of relevance reasoning based D -Separation in model structure mining,a junction tree relevance reasoning method is proposed.Finally,the validity and accuracy of the method is verified by experiment,providing an effective solution of precise reasoning under situation assessment.