The invention provides a green low-carbonic acid uranium leaching tailing filling material and a preparation method thereof, and belongs to the technical field of mine filling and low-radioactivity solid waste disposal. The filling material comprises uranium tailings, a cementing material, lime, a slump retaining agent and water, and the cementing material comprises cement clinker, phosphorus slag powder, slag powder and silica fume; and the raw materials are mixed through specific steps, so that the synergistic effect among the raw materials is enhanced to the maximum extent, and the fixing of nuclides and the strength of the filling material are improved to the maximum extent. The multi-source active solid waste is adopted as a main component of the filling cementing material, acid regurgitation of uranium tailings can be inhibited, the strength and later stability of a filling body are greatly improved, the cost is reduced, and release migration of radionuclides can be effectively controlled; the solid waste utilization rate of the uranium tailings in the filling material is high, the solid waste in the cementing material is low in value and wide in source, low-carbon and environment-friendly treatment of the uranium tailings can be achieved, and the filling material has good application prospects.