The invention relates to a method for testing a dielectric loss factor and electric capacity without dismounting a high-voltage lead of a transformer. After connection of wires at three sides of a transformer, the wires are connected to accessory equipment like a lightning arrester, a capacitive voltage transformer, an insulator string, and supporting insulators of the leads. According to the invention, attached capacitance values and dielectric loss values brought by the accessory equipment are measured separately; when no lead is dismounted, the tested data are calculated and transformed into real data of the transformer; the converted data are compared with original data to determine whether the transformer is in a normal state. Because the testing is carried out while no lead is dismounted, the risk of operation is reduced; and the induced electricity is controlled in advance.