As the current contains many harmonic components at both sides of the transformer, in the normal operation, the unbalanced current generated in differential protection is greater, which cause certain dispute over whether the excitation transformers should be equipped with differential protection. This paper analyses an example of the mal-operation of excitation transformer differential protection, and finds that high-order harmonic is not the root cause of the mal-operation. Mature operating experience of differential protection of excitation transformer has been accumulated, through proper setting calculation and configuration, the sensitivity of the protection for detecting internal faults of excitation transformer can be improved.