A fibre laser oscillator is constructed using the bidirectional-pumping structure and a 25/400 μm (the core diameter is 25 μm and the cladding diameter is 400 μm). The gain medium is a large-mode-area double-cladding ytterbium-doped fibre, and a 915-nm semiconductor laser is utilised as the pump source. The suppression of fibre-stimulated Raman scattering and dynamic mode instability is accomplished via fibre selection, an appropriate ratio of forwarding and backward pump power and mode control. The fibre laser oscillator produced a maximum output power of 5.08 kW at a pump power of 7.5 kW, with a light-light conversion efficiency of 68%, and the suppression ratio of stimulated Raman scattering is 37 dB. The time-domain properties remained constant, with no evidence of dynamic mode instability. The beam quality measurement results of output laser in the X and Y directions at maximum power are 2.483 and 2.514, respectively. The far-field light spot had a ring shape, and the light intensity ratio between the ring area and the central area is 1.6. The fibre laser oscillator worked continuously for 1 h at maximum output power without any abnormality, and the temperature of the fibre optic components in all parts is within the acceptable range.