The invention discloses a technique for processing a joint where box steel beams with relatively large slenderness ratios of sections are in butt joint. A rear plate is reserved on the side face for butt joint of the box steel beams, an upper flange plate and a lower flange plate are provided with dual connection lug plates and are fixedly connected by means of a connecting plate, flat welding and vertical position welding are used for positioning, so that the upper flange plate, the lower flange plate and two webs of each box steel beam are obtained, and then vertical position welding and horizontal position welding are used for welding a rear weld plate. By the adoption of the technique, the situation that potential quality hazards or defects are caused by over-head position welding occurring during installation of the box steel beams can be reasonably avoided; in addition, installation is convenient, a bolt connecting plate with holes does not need to be additionally arranged on the side face, and thus good economical benefits and good construction period benefits are obtained.