This paper provides an algorithm for minimum cut of a fault tree based on step function. According to analyzing the characteristic of the logic of “and” and “or” gates, this paper derives a conversion rule between the logic of fault tree and the step function. Using this conversion, we can convert fault tree to step function which is easy to calculate by programming in the computer. The amelioration of the algorithm can cut down calculated amount dramatically. The results of exemplified calculation demonstrate that minimum cuts of fault tree can be accurately and quickly obtained.