With the growing problems of mobile network calls, it is necessary to create a fault tree analysis (FTA) model. Using the BDD method, this paper analyses each basic event's structural importance, probability importance and critical importance. The order of the basic event of fault tree has a direct impact on the number of BDD nodes, as well as the structural importance and probability importance degree. With the neighbor-event-first method, the number of BDD nodes and the important degree of impact can be minimized. The importance degree with BDD method value is closer to calculate than the traditional FTA method. And the calculation is more efficient.