Abstract: Through coordinating the engineering system's present investment and its loss expecta-tion(represents the long term benefit of the engineering system),this paper proposes a method for the decision of its optimal reliability.This method bases on the rational apportionment for the engineering system's investment among its structures and is realized by letting each structure effectively suffer the loss of whole system.For this purpose,the re-lationship between the construction cost and the reliability of the whole system is reduced according to the corresponding relationships of its structures.