Auxiliary power unit (APU) is a smallsized engine installed on the aircraft which is used to provide auxiliary power resource. In the process of APU system safety assessment of civil aircraft, dynamic fault tree is gradually used to optimize the traditional fault tree, so that the fault tree analysis results can more accurately reflect the dynamic characteristic of system failure. Different from the traditional fault tree, it can be solved by Boolean logic; dynamic fault tree usually needs to be transformed into isomorphic state space model to solve. This kind of solution process lacks generality and has the problem of exponential explosion.In this paper, the discretetime TS dynamic fault tree calculation method was employed to calculate the failure probability of the top event of a subtree in APU system fault tree, and the results were compared with those calculated by Markov model. The results show that the relative error decreases monotonously with the increase of task time segments. When the task time segment is more than 5, the relative error is less than 1%, which can significantly reduce the calculation cost when the calculation accuracy meets the requirements.