The invention relates to an upper straightening mechanism of a bridge board unit straightening machine, which belongs to the technical field of mechanical equipment. The upper straightening mechanism is particularly used for straightening after a plurality of U ribs or board ribs are welded with a bottom plate unit, and mainly comprises I-beam guide tracks and a plurality of straightening mechanisms, and each straightening mechanism mainly consists of rollers, support frames, guiding supports, an oil cylinder, a large clamping roller, a speed reducer, a small clamping roller, an oil cylinder connecting plate and an adjusting bottom plate. Each oil cylinder is positioned between the corresponding support frames, a piston rod in each oil cylinder is sequentially connected with the corresponding oil cylinder connecting plate and the corresponding adjusting bottom plate, a first clamping roller seat and a second clamping roller seat are respectively connected to two sides of the lower end of each adjusting bottom plate, and the four guiding supports of each straightening mechanism are disposed at four corners among the support frames and the adjusting bottom plate of the straightening mechanism. The upper straightening mechanism is simple, compact and reasonable in structure, board unit straightening efficiency and straightening precision are greatly improved, and automatic straightening is realized.