The invention discloses a capacity-adjustable transformer monitoring device. The device comprises a box, wherein a first groove and a second groove are formed in the box, capacity adjusting switches, mounting belts and a positioning rod are arranged in the first groove, through holes are formed in the mounting belts, limit barrels, screws and first bolts are arranged on the positioning rod, fixing rods and a transformer are arranged in the second groove, bolt grooves are formed in the fixing rods, connecting blocks and second bolts are arranged on the transformer, a mounting block is mounted at the bottom of the box, a left box door, a right box door and binding posts are arranged on the box, hinges are arranged on the right box door, a binding hole and a counterbore fixing groove are formed in the right box door, a voltmeter and an ammeter are arranged in the counterbore fixing groove, and nuts are arranged on the binding posts. With the adoption of the capacity-adjustable transformer monitoring device, the production cost can be reduced, the safety performance can be greatly improved, the electric quantity of the transformer can be monitored very directly, and the device is simple in structure and easy to implement.