Abstract: This paper applied the Monte-carlo simulation, based on the Bayes method, to the FTA. Then a new method to obtain the data of the failure probability of a bottom event was proposed. It overcame the difficult in the quantitative analysis due to the insufficient data of event regarding the safety and therefore, solved the problem with less data resources. Finally, this new method was used to the safety analysis for a hearth on board, and some quantitative results were obtained together with some adviees.