According to a cracking accident of an incoming cable GIS terminal epoxy casing in a substation main transformer,the macroscopic crack morphology,the material performance and the mechanical simulation of epoxy casing are analyzed in this paper.Thinking that the root cause of epoxy casing cracking is a large installation deviation of the connection conductor and the cable terminal.So that the groove of the epoxy casing flange is operating in a state of stress close to its material strength limit for a long time,and the safety margin is insufficient.Accordingly,replacement recommendations are presented to ensure that the installation deviation should not be greater than 0.5 mm.