Abstract: Since the multiple evidences from different sources have different significances，the concept of evidence weight is introduced to improve the precision of data fusion． Based on the essential relation between the rough set theory and the evidence theory ，a new method with the weighted D-S evidence combined is proposed．The instance of survivability assessment on the water fire fighting system of warships is used to validate the proposed method．It is shown that the method can improve the precision of fusion result and make the process of fusion in a more logical manner．It provides a new way to assess the design of the water fire fighting system of warships．