The invention discloses a multi-layer composite insulating material and a preparation method thereof. The multi-layer composite insulating material comprises industrial rigid laminated board layers of electrical thermosetting resin and oft composite material layers, wherein the industrial rigid laminated board layers of the electrical thermosetting resin and the soft composite material layers are alternately stacked; the industrial rigid laminated board layer of the electrical thermosetting resin are arranged on an upper outer surface layer and a lower outer surface layer of the multi-layer composite insulating material; and the total number N of the layers is greater than or equal to 3. The preparation method of the insulating material comprises four steps of material preparation, material arrangement, hot-pressing, and cooling and demolding. The multi-layer composite insulating material disclosed by the invention has excellent mechanical property and excellent voltage resistance; the preparation technology is simple; and the multi-layer composite insulating material can be applied to different fields according to actual requirements.