The invention belongs to the technical field of comprehensive pipe racks, and particularly relates to a quasi-rectangular comprehensive pipe rack pipe piece connection structure and an assembling method. A produced comprehensive pipe rack can be used as an underground urban pipeline comprehensive pipe rack. The connection structure comprises arc angle pipe pieces, plane pipe pieces and vertical face pipe pieces. According to the specific structure, the arc angle pipe pieces, the plane pipe pieces and the vertical face pipe pieces are prefabricated pipe pieces, the quasi-rectangular comprehensive pipe rack is formed by splicing the prefabricated pipe pieces, the plane pipe pieces are arranged on the upper and lower portions of the comprehensive pipe rack, the vertical face pipe pieces are arranged on the left and right portions of the comprehensive pipe rack, and the four corner points of the comprehensive pipe rack are provided with the arc angle pipe pieces with arc sections. In the connection process of the quasi-rectangular comprehensive pipe rack, the defect of welding is avoided, construction operation is greatly simplified, and the construction period can be greatly shortened. The quasi-rectangular shape is more beneficial to space structure division, the space utilization rate is improved, the shape is simple, the width and height can be changed according to undergroundspace requirements, and the quasi-rectangular comprehensive pipe rack can be embedded more deeply compared with the same underground depth.