Power transformer is one of the most important equipments in power transmission, and it is of great significance to detect the aging state of transformers. In order to effectively evaluate the aging condition of oil paper insulation in transformers, the dielectric properties of oil-paper insulation under different aging degrees are studied by frequency domain spectroscopy (FDS). Besides, the relationship between the characteristic parameters and aging time is established by Havriliak-Negami (H-N) model. Then, this method is applied to the field transformer test. The results show that with the deepening of aging degree, the FDS curve increases significantly in the middle frequency band, but does not change significantly in the high and low frequency bands. The overall curve is in the shape of 'jujube nucleus'. According to the H-N model, the characteristic parameters have a good correspondence with the aging degree. This method that evaluates the aging condition of transformers is verified by the application of the field transformer.