The invention relates to an indicating device and a preparation method thereof. The indicating device includes a bottom plate layer, an indicating layer, a phase change material, a phase change material storage chamber, and a liquid phase channel, and the bottom plate layer comprises, at least five layers, a first interlayer, a first adhesive layer, a sandwich layer, a second adhesive layer and a second interlayer that are successively laminated. During hot pressing, the sandwich layer is protected between the first interlayer and the second interlayer, and the bottom plate layer is not easy to break by attaching the first interlayer to the indicting layer of an observed area. The preparation method of the indicating device is simple in operation, and the prepared bottom plate layer of the indicating device is not easy to break.