Aimed at the most unwilling top event of system， fault tree finds out the all factors that cause the top event through analysis. In fault tree analysis， two binary decision diagrams (BDD) is the most efficient method. As both fault tree and causality diagram can express causation by digraph， there are lots of similarity in common. However， BDDis widely used in Fault tree. The research shows that under certain conditions， fault tree and causality diagram can be transformed into each other. Therefore， causality diagram can be analyzed by BDD on the basis of BDD principle.