Abstract: In the integrated energy system, heterogeneous energy sources exhibit quality differences, and each subsystem possesses different transmission characteristics. Therefore, traditional reliability evaluation methods are not fully applicable to integrated energy systems. To address this, an integrated energy system reliability assessment method is proposed, taking into account both energy quality and multiple states of failure. Firstly, an expected unsupplied exergy index is introduced considering the users’ requirements in terms of qualitative and quantitative aspects. The traditional evaluation index for energy supply reliability is then adjusted to incorporate the transmission delay of heterogeneous energy, based on the criterion of whether the energy supply quality can meet user expectations. Furthermore, we establish a multi-state transition model for the dynamic characteristics of the gas and thermal transmission processes. Finally, the effectiveness of the proposed method is verified through simulation examples, and the impacts of the multi-state failure transition model and the revised reliability indicators on the reliability assessment results of integrated energy systems are analyzed.