The invention provides a multimode fiber. The multimode fiber includes a core layer, a sinking cladding layer and an outer cladding layer; the core layer is composed of a gradual change layer and a platform layer, the core layer is a glass layer formed by blending GeO2 with other doped materials, and the molar concentration distribution of the doped materials is shown in the following function which can be seen in the description, wherein M(r) refers to the molar concentration of the doped materials in the position wherein the radial distance between the doped materials and the center axis of the core layer is r, Mo refers to the molar concentration of the doped materials in the center of the core layer, Mb refers to the molar concentration of the doped materials on the edge of the core layer, and beta refers to the concentration distribution parameter of the doped materials. The multimode fiber is very low in bending loss when the bending radius is small, can support data transmission of a wide-wavelength-section window, effectively improves the transmission capacity of optical fibers and is compatible with traditional multi-mode optical fibers.