An approach to construct a disjoint tree for an event set is presented, in which the total number and order of terms of a cut set (path set) are decreased rapidly through the separation of independent terms and the collection of common factors. All of the branches of the disjoint tree are disjoint to one another, consequently,the occurrence probability of the top event can be obtained straightforwords. Application examples are given. The extension of the approach to the multistate systems and the approximate calculation method are discussed.