Modeling on the basis of reliability theory of multi-state element overcomes the shortage of traditional binary reliability model of information loss,and improves the accuracy of assessment.The existing models mainly focus on the analysis of elements with gradual failures.After Markov processing is introduced,definition of multi-state element is given in detail.The continuous-time Markov processing in element state-transition is described,based on which the state-transition reliability models for both gradual failures and mutational failures of elements are proposed.Taking the delay valve of a certain type of missile as an example,the state-transition differential equations with gradual failures and mutational failures happening simultaneously are established,and the reliability functions of the delay valve at a certain demand level are determined.The reliability of the element at determined moment is obtained,which can supply a reference for making maintenance and support decisions.