The invention belongs to the technical field of bridge engineering measurement equipment, and specifically relates to a bridge box girder section thickness measuring instrument. The instrument comprises a straight ruler, a circular angle ruler, base rulers, a clamping block, a brake head, a control button, a transmission cable, a spring-containing rotating shaft, and movable claws. The control button is arranged at one end of the straight ruler, and the other end of the straight ruler is connected with the movable claws through the spring-containing rotating shaft. The clamping block is arranged in a sliding manner on the straight ruler, and is connected with the brake head. One ends of the two base rulers are connected with the brake head, and the other ends are respectively connected with the bottom edges on the two sides of the circular angle ruler. The two movable claws are connected to the control button through the transmission cable. Compared with the prior art, the bridge box girder section thickness measuring instrument of the invention can be used for accurately measuring the thickness of the section of a bridge box girder, and has the characteristics of being simple to use, capable of adapting to various tilt angles, intuitive in reading, low in processing cost, and the like.