The angle-polished side-pump coupler is fabricated by fused splicing angle-polished multimode fibers and 20/400 μm double-clad Yb-doped large mode area fiber together. The coupling efficiency of side-pumped coupler for 975 nm pumping light can reach up to 97%, and the leakage ratio of 1080 nm signal light is less than 2%. We analyze the performance of a single side-pumped coupler and also the influence of the distribution of cascade stage couplers on the efficiency of fiber amplifier. Using self-developed side-pumped couplers, we build a distributed side-pumped, ytterbium-doped double-clad all-fiber master oscillator amplifier, and an continuous and single mode laser of 1080 nm wavelength with a 303 W maximum output power is obtained. Finally, increasing the number of the side couplers to improve pump power, the output power of the amplifier becomes higher.