The invention relates to a method for judging DC magnetic bias in a 330kV single phase autotransformer. In the method, equivalence is carried out according tomagnetic flux conservation principle under the same front-and-back input current. In the method, a DC current at a neutral point of a 330kV autotransformer and a DC current at a neutral point of a transformer with the next voltage level are collected. When the neutral point of the 330kV single phase autotransformer is put into a DC blocking device, the DC current passing through each phase winding in the 330kV autotransformer is obtained by equivalence based on the magnetic flux conservation principle. According to the requirements of HVDC grounding electrode technical guidelines, the DC current passing through each phase winding in the 330kV autotransformer is judged to whether exceed standard or not. With the help of modeling of the 330kV single phase autotransformer, equivalence is carried out according to magnetic flux conservation principle under the same front-and-back input current, problems for measuring DC current in series windings of the autotransformer after the neutral point of the 330kV autotransformer is put into a DC blocking capacitor are overcome, and reliability of normal operation of the 330kV autotransformer in need of bearing DC magnetic bias is ensured.