The invention relates to a method for preventing fuel oil from leaking in a fuel oil pressure test. A leakage preventing device comprises a test rod, a gland, a top core and a valve, wherein the inner wall of the test rod is provided with an upper seal ring and a lower seal ring along the circumferential direction; the upper part of the top core is connected with the gland to be a whole; the lower part of the top core is inserted into the inner cavity of the test rod; the upper seal ring covers the top core; the valve which is in a frustum shape is arranged below the top core; a spring is installed between the bottom of the valve and the bottom of the test rod and enables the conical surface of the valve to be glued with the lower seal ring; an oil duct is arranged inside the gland and the top core, and penetrates through the gland and the top core; the inlet of the oil duct is arranged on the side wall of the bottom of the top core, and the outlet of the oil duct is arranged on the gland; when the gland is connected with the test rod, the top core moves downwards to push the valve so as to separate the conical surface of the valve from the lower seal ring; the lower seal ring covers the top core; and the inlet of the oil duct on the side wall of the bottom of the top core is communicated with a cavity in which the valve is arranged. The device can ensure no fuel oil leakage when in test.