The invention discloses a preparation method of a calcium carbonate based reinforcing agent for a PVC (polyvinyl chloride) advertising film. The preparation method fundamentally comprises the steps of carrying out clean-washing, air-drying and smashing on calcite raw ore; grading by a vibrating screen; mixing for preparation; coating; drying; and modification. The calcium carbonate based reinforcing agent has the beneficial effects that the tensile strength of the PVC advertising film added with the reinforcing agent prepared by the preparation method is improved by more than 10% compared with that of the PVC advertising film added with common heavy calcium carbonate with the same quantity as the reinforcing agent, both the elongation at break and the product glossiness are improved remarkably, an antibacterial characteristic is added, and the filling content can be improved 5% under the condition of the same product quality.