A molybdenum diselenide (MoSe.sub.2)/InGaN multispectral photoelectric detector includes a substrate, a buffer layer, an InGaN layer and a MoSe.sub.2 layer that are arranged sequentially from bottom to top. The MoSe.sub.2 layer partially covers the InGaN layer. The photoelectric detector further includes a barrier layer and an electrode layer. The barrier layer is provided on the InGaN layer not covered by the MoSe.sub.2 layer and on a part of the MoSe.sub.2 layer. The electrode layer is provided on the barrier layer and covers a part of an exposed portion of the MoSe.sub.2 layer. A preparation method of the detector is further provided. The detector detects red light and blue light at the same time. While realizing a sensitivity enhanced micro-nano structure on a surface of a detector chip, the detector improves quantum efficiency in blue and red bands, and enhances resonant absorption for the blue light and red light.