The invention discloses a large-span assembly type open-type construction bottom plate I-beam mounting support seat and mounting assembly and belongs to the technical field of construction engineering. A mounting support seat comprises support plates, a vertical plate and an assembly part. The side faces of the two support plates are arranged opposite to each other, and the two ends of the bottom of the vertical plate are fixedly connected with the upper surfaces of the support plates. The opposite side faces of the two support plates and the bottom surface of the vertical plate form a limiting groove that the width of the limiting groove is not less than the width of an upper flange on an I-beam. Through holes are formed, outside connected with the vertical plate, in the support plates. The assembly part is a combination of a circular platform and a bending plate with an L-shaped cross section, the side face of a free end of a long side of the cross section of the L-shaped bending plate is fixedly connected with the side face of the circular platform, and the corner of the L-shaped bending plate cross section is a circular corner structure. The bottom surface of the circular platform of the assembly part is provided with a threaded hole which penetrates through the circular platform and the support plates and communicates with the vertical plate, and meanwhile, the mounting assembly containing the support seat is also provided. The integrity of the construction bottom plate can be effectively maintained due to no need for destructive treatment such as hole punching, and the service life of the construction bottom plate can be prolonged.