The invention discloses an oil level fault diagnosis device for an oil-immersed power transformer, so that a problem that the existing oil level indicator fails to monitor an oil surface of an oil-immersed power transformer in operation can be solved. A transformer oil tank (1) is arranged on a transformer in operation; a transformer casing tube (2) and a transformer expansion tank (3) are arranged on the transformer oil tank (1); and an oil level indicator (4) is arranged on the transformer expansion tank (3). The device is characterized in that an oil level testing box (5) is arranged on the plane at the outer side of the transformer expansion tank (3); a right oil level measuring dial gauge (13) and a left oil level measuring dial gauge (14) are arranged on the oil level testing box (5); a vibration sensor (7) is arranged on the right oil level measuring dial gauge (13) movably and connected with a vibration frequency monitoring instrument (10) electrically by a signal transmission line (9). According to the invention, testing and diagnosis can be carried out on the premise that the power of the transformer is not interrupted.