Abstract: In order to optimize the design of M3 fuel and further improve its reliability in LWR environment, it is necessary to research its failure mechanism under steady-state operation conditions. Based on the ABAQUS software, a 3D numerical simulation analysis method for the irradiation-thermal-mechanical coupling behavior of M3 fuel was established by the way of secondary development. With the help of this established analytical method, the failure mechanism of M3 fuel under steady-state operation conditions was carried out. According to the research results, during the steady-state operation conditions, the failure mechanism of M3 fuel is mainly dominated by the failure of IPyC layer in the initial irradiation and the failure of SiC layer caused by the contact of Buffer layer and IPyC layer. This research results could provide guidance for the optimization design of M3 fuel.