The invention discloses a method for measuring the dielectric loss, the capacitance and the insulation resistance of a transformer without disassembling drainage wires, and relates to the technical field of high-capacity transformer overhauling. Drainage wires connected with the high-voltage winding, the medium-voltage winding and the low-voltage winding of the transformer do not need to be disassembled from a drainage plate, iron core ground connection is cut off, clamping element ground connection is cut off, a shell is under a ground connection state, and after two parts to be tested are independently subjected to the short circuit, the two parts to be tested are connected with a testing instrument through test wires so as to measure the winding as well as the casing pipe insulation resistance, the dielectric loss and the capacitance. The method has the beneficial effects that the method adopts a forward connection method, defects brought by disassembling the drainage wires are overcome, and meanwhile, data obtained by testing is accurate. The method saves manpower, the test method is optimal, working efficiency and safety are improved, a complex process of drainage wire disassembling is avoided, and one series of problems which may be caused by drainage wire connection can be eliminated.