Fault Tree Analysis (FTA) is a traditional deductive safety analysis technique that is applied during the system design stage. However, traditional FTA does not consider transitions between states, and it is difficult to decompose complex system fault events that are composed of multiple normal components? states rather than individual component failures. To solve these problems, we first propose two different fault events of fault trees, and then present a formal fault tree construction model by introducing the concept of transition rules for event decomposition, in which the semantics of gates and minimal cut sets of fault trees are revised compared with traditional FTA.