At present,the research on on-line diagnosis of electronic transformer soft fault is rarely seen.Aiming at line electronic transformer,the current and voltage models of the drift bias fault and transformer bias fault is established based on the physical temperature characteristics of components.The double terminal six current and single terminal four voltage of line fiber differential protection are used to set up the electronic transformer soft fault diagnosis criterions,the output current and voltage of double protection device are compared with vertical and horizontal to find the fault electronic transformer.The method could realize on-line fault diagnosis of electronic transformer on existing protection under the condition of no additional hardware and ECT without power failure and online.Matlab simulation result shows that the method is corrected and validated.