In this study, a 550-W single-frequency all-fiber laser with near diffraction-limited beam quality is achieved by simultaneously suppressing the stimulated Brillouin scattering and mode instability effects with a long-tapered gain fiber and employing the ability of high mode instability threshold of 1030-nm signal laser. The slope efficiency of the whole amplifier is observed to be as high as 80% and the measured beam quality M2 is approximately 1.47 at the maximum output power. Further brightness scaling is limited owing to the mode instability effect.