The invention discloses an oil liquid level monitoring device of a transformer. The oil liquid level monitoring device comprises a first vertical rod fixedly hung on the top face of an oil tank, a second vertical rod fixedly hung on the top face of the oil tank, a floating block arranged on the first vertical rod in a sleeving mode, bar codes drawn on a rod body of the second vertical rod in a zero adjacent interval mode, a bar code scanner arranged on the top face of the floating block and aligned with the bar codes, and a computer used for displaying the reading obtained through the bar code scanner. The first vertical rod and the second vertical rod are parallel to each other and are of the same length. The top ends of the first vertical rod and the second vertical rod are both detachably connected to the top face of the oil tank. The bottom ends of the first vertical rod and the second vertical rod are both inserted in oil. The number of the bar codes is equal to the number of scales of a length ruler. The oil liquid level monitoring device has the advantages that the oil liquid level monitoring device is compact in structure, convenient to disassemble and assemble, safe and reliable; efficient online monitoring is achieved; oil liquid level changes of the transformer can be reflected in real time; potential faults and fault development tendency of the transformer can be conveniently found in time, and therefore maintenance personnel can easily and scientifically arrange the maintenance plan of the transformer, accidents are reduced, and the maintenance cost is saved.