The invention discloses a special fixture for assembly and welding of angle iron and a multi-rib plate flange port. The special fixture comprises a base; a plurality of supports are fixedly arranged at the top of the base; a bearing plate is fixedly arranged at the tops of the supports; a plurality of third rotating air cylinders are fixedly arranged at the top of the bearing plate; rotating armsare arranged on output shafts of the third rotating air cylinders; quenching blocks are arranged on the rotating arms, and are located above the bearing plate; a protection mounting pipe is fixedly arranged at the top of the base; self-adapting compressing components are arranged on the protection mounting pipe; first locating blocks are arranged at the top of the base, and are adapted to the self-adapting compressing components; and angle iron locating pin assemblies and 3B angle iron backing assemblies are arranged at the top of the base. Angle iron and a flange can be clamped and fixed conveniently, the time and energy are saved, the labor cost is lowered, and the special fixture is simple in structure and convenient to use.