In this paper, we have experimentally demonstrated a high power and high brightness narrow-linewidth fiber amplifier seeded by an optimized fiber oscillator. In order to improve the temporal stability, the fiber oscillator consists of a composite FBG-based cavity with an external feedback structure. By optimizing the forward and backward pumping ratio, nonlinear effects and SRS-induced mode distortion of the fiber amplifier are suppressed comprehensively, accompanying with the simultaneous improvement of beam quality and output power. Furthermore, the TMI threshold is also improved by ~1.0 kW by coiling the gain fiber with a novel curvature shape. Finally, a 6 kW narrow linewidth laser is achieved with beam quality (M2) of ~1.4. The laser brightness has doubled comparing to the results before optimization. At the maximum output power,. To the best of our knowledge, it is the highest brightness narrow linewidth fiber laser based on one-stage MOPA structure.