The invention relates to a method for carrying out dielectric loss and capacitance measurement on a transformer without detaching incoming cables. The method is characterized in that the method comprises the following steps: a dielectric loss tester, a testing transformer and a tested transformer are connected together; during testing, the cables are connected in series with a dielectric loss tester standard capacitor to serve as a test loop standard capacitor, the transformers form a test loop, the transformer dielectric loss value is obtained by comparing phase relation of standard loop and test loop signals, and the transformer capacitance can be obtained through calculating; and the transformer, of which the dielectric loss and the capacitance are determined, is used as a standard capacitor to form a standard loop, the cables serve as the test loop, and the dielectric loss and the capacitance of the cables can be calculated according to the dielectric loss and the capacitance of the transformer. According to the method, the dielectric loss and the capacitance of the transformer and the cables can be obtained simultaneously without detaching the high-voltage lead wires, so that the purposes of safe operation, normal maintenance and manpower and material resources saving are achieved; and the method has the advantages of being high in test work efficiency, safe and reliable and convenient to use and the like.