Abstract: This technology comprises a combined accessory of an in-situ concrete 3-D printed horizontal load-bearing member and a preparation method. The combined accessory includes 3-D printed concrete and a bottom mesh, the 3-D printed concrete contains fine aggregates having the particle size of 0.08 mm-4.75 mm, and the fluidity of the 3-D printed concrete is larger than or equal to 110 mm and smaller than or equal to 190 mm; the bottom mesh is a reinforcing mesh or expanded metal, the diameter of the reinforcing bar is larger than or equal to 0.5 mm, and the mesh aperture of the reinforcing mesh is smaller than or equal to 7.5 times of an upper limit of the particle size of the fine aggregate and is larger than or equal to 7.5 times of a lower limit of the particle size of the fine aggregate.