To ensure the reliability of construction system, this paper proposes an enhanced factoring method. Firstly, it finds out the isomorphic sub-networks from decomposition and decreases recomputations. Secondly, it stores the network states in OBDD (ordered binary decision diagram) and decreases redundant states. These steps enhance the factoring method and make it easy to compute network reliability for large construction system of engineering project. The experiments show this method is efficient for large project.