Abstract: The distribution network system is a neutral point non effetaive earthing system,the short circuit current is small when single-phase grounding fault occurs, and there are many distribution network lines and their branches, it is difficult to configure relay protetaion,we often only considers a single fault when the power network fault, and pays little attention to the problem of composite fault. A detailed analysis was condutaed on the main transformer protetaion ataion caused by distribution network line heteronymous phase cross country fault, the hidden cause of the fault was identified, and solutions were proposed based on the existing problems, providing reference for on-site distribution network fault analysis and operation and maintenance.