The invention provides a large-mode-field all-solid-state optical fiber, and the optical fiber comprises an active fiber core and a cladding layer. The active fiber core is located at the center of the optical fiber, and a plurality of microstructure units are arranged in the cladding layer. The microstructure units are arranged through taking the active fiber core as the center of symmetry, and each microstructure unit comprises multiple reflection layers which are distributed from the inside to the outside, wherein the refractive indexes of the reflection layers change alternately. The invention also provides a preparation method for the large-mode-field all-solid-state optical fiber. The refractive index annular structure of the microstructure units is similar to a double-layer Bragg reflector, and can enable a basic mode to be limited in an active fiber core region for conduction. Through the adjustment of the number, period, size and reflectivity of the microstructure units, the method can improve the leakage loss of a higher-order mode, and achieves the design of a large-mode-field area single-mode optical fiber. According to the invention, the design dimension of the microstructure units in the cladding layer is increased, and the controllability of the conduction mode of the active fiber core is improved. The new design mode facilitates the implementation of the batch controllable preparation of large-mode-field active optical fibers.