The waveguide structure of the large-mode microstructure fiber is designed in view of the efficient coupling and laser output quality during the practical application of the high-power fiber laser. It employs the bundle drawing technology to prepare the ytterbium-doped microstructure fibre with the core diameter of 41 μm, inner-cladding numerical aperture of 0.62, core numerical aperture of 0.05 and effective mode area of 530 μm2. The output power of the single-mode laser is 19.1 W, the slope efficiency is 55.2% and the beam quality factor of M2 is less than 1.01, on the condition that the pump power is 35.0 W.