The switching process of on － load tap changer ( OLTC) is introduced． The oil color spectrum analysis，short － circuit impedance，winding deformation，winding DC resistance and the switching time and action sequence of on － load tap changer are tested based on the fault of on － load tap changer of a 110 kV transformer． The cause and process of the accident are analyzed according to those test results combined with the inspection of hanging cover，and a new preventive detection method and new organizational measures are given for the accident．