The invention provides a PLA (poly lactic acid) sheet and a preparation method thereof. The PLA sheet is produced by adopting a three-layer co-extrusion process and is of an A/B/A structure, layers A are composed of PLA resin, a flux-strength enhancer, a light stabilizer and an antioxygen, and a layer B is composed of PLA resin, a flux-strength enhancer, a nucleating agent and an antioxygen. The PLA sheet has the advantages that the PLA sheet can take the place of PVC (polyvinyl chloride) to be used to produce smart cards, and the PLA sheet is good in heat resistance, capable of being wound, free of warping in card making and the like; the preparation method is simple, and mass production is facilitated.