The invention discloses a smart power grid on-line fault diagnosis method based on a GEP algorithm. The method involves a transformer fault diagnosis method based on the GEP algorithm and a fault diagnosis device based on the diagnosis method. The transformer fault diagnosis method includes the steps that gas concentration data are normalized, five commonly-seen states of a transformer are classified through seven GEP classifiers, and therefore whether the transformer has a fault or not is judged and the specific fault type is judged. According to the method, an accurate result can be acquired according to limited training samples and the accuracy of fault diagnosis is improved.