The invention discloses a large mode field area and microstructural optical fiber with the mode field distribution of a flat base mode, which relates to a microstructural optical fiber and solves the problem of high-precision control for production technology that a traditional large mode field area optical fiber with the mode field distribution of a flat base mode is provided with an ultralow optical core number hole. The microstructural optical fiber is composed of an optical core and a cladding which surrounds the optical core, wherein a core zone is composed of one or seven air hole defects; the cladding is composed of air holes (2) arranged in a triangle period, or fluorine-silica medium cores (4); and a germanium-doped silica medium layer which is provided with a specific radius andrefraction index of inner layers and outer layers is led in the juncture of the fiber core of the microstructural optical fiber and the cladding. The base mode field of the microstructural optical fiber has the characteristic of flat distribution, and has very important action on improving the output power of a large power optical fiber laser or an amplifier, particularly on inhibiting the nonlinear effects in a high-energy pulse optical fiber laser or an amplifier.