Modular outfitting method plays an important role in shipbuilding, including installation of engine room machinery to improve the effectiveness, save man-hour and have bigger competitive in the world. It is a very important subject to user and shipbuilder how to guarantee the reliability of power plant from the start of shipbuilding, i.e. how to distribute reasonably reliability to each module because the different module has its' different role in system. In the paper a new method to determine importance degree is presented. The fuzzy function in this method is used to determine the importance degree because it is difficult to precisely determine the parameters of probability distributions and detailed data. The method proposed in this paper also can be used in other engineering calculation.