A reinforcing steel bar welding slag collecting device relates to the technical field of constructional engineering construction and comprises a cuboid box body and a cover plate, the box body is provided with a front end face, a rear end face, an upper end face, a lower end face, a left end face and a right end face, an embedding groove vertically penetrates through the box body, and a side opening of the embedding groove extends to the rear end face; one vertical side edge of the cover plate is hinged to the rear end face of the box body. The upper end and the lower end of the cover plate are bent in the same direction and cover the rear end face, and bent parts are attached to the upper end face and the lower end face respectively. Notches are formed in the bent parts of the upper end and the lower end of the cover plate, the notches and the upper end openings and the lower end openings of the embedding grooves form steel bar embedding holes in the vertical direction, sliding grooves in the vertical direction are formed in the left end faces, the handle portions of the steel wire brushes are clamped in the sliding grooves in a sliding mode, and brush heads of the steel wire brushes extend into the space between the two steel bar embedding holes. Welding slag splashed during welding can be collected in a centralized mode, meanwhile, the welding slag adhering to a weld joint is cleaned up, operation is flexible and easy, and the safety of constructors is guaranteed.