UHV AC/DC interconnected power grid is the main form of power grid in the future，and large capacity transformers are widely used． When the main transformer of a new substation is put into operation or put into operation after maintenance， large inrush current will appear in the transformer with residual magnetism，which has an important impact on AC and DC power grid． In the long － line weak － link power grid，the inrush current and sympathetic inrush current generated by the transformer with residual magnetism will lead to the disoperation of the transformer differential protection and the harmonic voltage for a long time，which is an important hidden danger of the security and stability for power grid． Therefore，it is necessary to analyze the generating mechanism of residual magnetism in power transformer，explore the detection method of residual magnetism，and eliminate the influence of residual magnetism on power grid． The demagnetization test is carried out for the transformer，and based on the frequency response of winding，the residual magnetism detection method has been studied． The research results show that: by comparing the frequency response in a certain sweep frequency range，it can accurately determine whether the transformer contains residual magnetism，and can determine the magnitude of the residual magnetism content of the transformer．