Calculating Top-Event failure probability is an important aspect of non-coherent FTA (Fault Tree Analysis). Because of the complexity of non-coherent system, all existing algorithms can't satisfy the requirements of exactness and speediness in application. In this article, we adopt a new algorithm for calculating Top-Event failure probability of non-coherent fault tree, which is based on BDD to decompose Top-Event of the fault tree and avoid seeking PIS and the smallest cover of PIS. So the new algorithm provides a new method for solving the NP problem effectively.