Abstract: Taking 300MW turbine generator as an example, the paper studied the natural characteristics of the interaction of magnetic saturation and magnetic field distortion and its influence on operation under critical instability condition by means of finite element method.By comparing and analyzing magnetic field distribution,magnetic flux density waveform, inductive electric potential and so on under different saturation degrees and power angles,it is found that their interaction not only causes changes in resultant magnetomotive forces producing the same air-gap potential,but also leads to the prominent changes in symmetry of magnetic flux density waveform and its harmonics content.Further research is done about the influences of these changes on the field current,the direct of air-gap magnetic density,potential waveform and magnetic flux density of stator teeth under the given load conditions.The above results can be helpful to further study on models and parameters of generators under abnormal conditions.