Abstract: It is difficult to apply the traditional fuzzy reliability theory to a practical engineering case because some fuzzy work states are introduced. The main reason is explained, that is, the method of dealing factors of some fuzzy work states is detailed continuously, the analysis model is complicated more and more and the calculation program is over elaborate, especially to on-off engineering system with complex logistic relation. In fact, a system and its elements all may be simplified as those with three-stage work modes of safety, intermediate and failure. The intermediate states are the trouble with a system and its elements. Naturally, a system and its elements are taken with three types of work states, that is, safety, intermediate and failure. A reliability analysis method of structures and systems with work mode of safety-intermediate-failure is introduced in the paper. Further, it is considered that an on-off engineering system and its structures all should have three work states of safety, intermediate and failure. The results of reliability analysis should be safety probability (reliability) P_R , intermediate probability P_M and failure probability P_F of the on-off engineering system, which compose so-called reliability vector P_R,P_M,P_F. The calculation model of reliability vector for the on-off engineering system is advanced. Finally, a simple numerical example is given out。