The invention provides a single-mode high-power amplifying device based on large-mode-field optical fibers. The device comprises a pump source, a coupling lens group, a first dichroic mirror, a seed source, a collimating lens group and a second dichroic mirror, wherein the pump source, the coupling lens group and the first dichroic mirror are sequentially arranged in the direction of a pump optical path, the seed source, the collimating lens group and the second dichroic mirror are sequentially arranged in the direction of a seed optical path, and the optical paths overlap at the large-mode-field gain optical fibers. Seed light can satisfy the collimated and non-wall-collision propagation in fiber cores of the large-mode-field gain optical fibers under a certain length after being collimated by the collimating lens group, but pump light forms waveguide transmission in the fiber cores of the large-mode-field gain optical fibers under the function of the coupling lens group, the seed light cannot be subjected to waveguide transmission in the fiber cores but can keep in a mode of amplified output, and meanwhile a great gain can be formed for the seed light through the waveguide transmission of the pump light; through the adoption of the large-aperture large-mode-field gain optical fibers, lasers have a large mode-field area in the amplification process, the peak power per unit area is reduced accordingly, and the effect of inhibiting the nonlinearity of the optical fibers is achieved.