Focusing on the difficulty of 1 018 nm ytterbium-doped fiber laser, the suppression of amplified spontaneous emission（ASE）, a scheme is proposed to realize the suppression of ASE, which based on the master oscillator power amplifier, combined with the large-mode-field double-clad ytterbium-doped fiber. Fiber length and pumping parameters are optimized, narrow linewidth laser output of 50 W and light-to-light conversion efficiency of 43.6% are realized. The amplified spontaneous emission and self-oscillation are suppressed efficiently, while the side-mode suppression ratio is 40 dB. The output power instability is measured in the experiments with the value of RMS approximately equal to 0.50%, V_pp approximately equal to 2.02% in four hours. The fiber lasers can be used in second harmonic generation and optical parametric oscillation.