The invention relates to an assembly type outer wall system of a steel structure, and belongs to the technical field of outer walls. The assembly type outer wall system comprises an assembly type steel frame main structure, wallboard connecting pieces, separate joints of connecting pieces on a beam, outer wallboards, a heat preservation layer and a decoration surface. The main structure comprises steel beams and steel columns. The connecting pieces are L-shaped connecting pieces. The joints and the connecting pieces can be welded to be a whole in a factory. The connecting pieces and the wallboards are connected through bolts. The joints and the main structure rapidly reach positions through the connecting pieces on the beam. The wallboards, the heat preservation layer and the decoration surface are connected through adhesives special for the heat preservation layer or mortar. According to the assembly type outer wall system for the steel structure, the welding portions of the connecting device can be completed in the factory, on-site assembling is conducted through the bolts, fastening work of the bolts is convenient and efficient, the construction speed is increased, the construction quality is improved, and the system is adapted to the development of the assembly type steel structure; meanwhile, effective connection between the wallboards and the steel frame main structure can be ensured through the joints of the assembly type outer wall system, and the wallboards are prevented from being disengaged.