Profile steel reinforcement and construction method for existing building stairs

The invention relates to an existing building stair reinforced by profile steel and a construction method thereof. The existing building stair comprises a combined steel beam, an I-shaped steel beam, anchor bolts, batten plates, newly-added channel steel, newly-added end channel steel and U-shaped connecting ribs. The combined steel beam is composed of channel steel and batten plates. The end part of the combined steel beam is welded with the I-shaped steel beam; the I-shaped steel beams are welded with the vertical ribs of the original platform beam frame through U-shaped connecting ribs; the newly-added channel steel is arranged on the two sides of the lower portion of the inclined plate of the original stair interlayer stair flight. The newly-added channel steel is connected with an original stair interlayer stair section inclined plate on the upper portion through anchor bolts; the upper end part of the newly-added channel steel is welded and fixed with the U-shaped connecting rib; the newly-added end par