A high power level ytterbium doped all-fiber laser at 1018 nm is proposed using the resonant cavity with the 125 μm - diameter double cladding fiber. The single mode high output power of 254 W is achieved with the optical-optical conversion efficiency of 81% via optimizing the length of the gain fiber and controlling wavelength of the pumping LD. The amplified spontaneous emission (ASE) is suppressed efficiently and the residual pumped light is dissipated completely, while the signal to noise ratio (SNR, RSN) is more than 35 dB. As the wavelength of the pumping LD changes with the output power, thus affecting the conversion efficiency of the fiber lasers, the wavelength of pumping LD needs to be controlled accurately for obtaining more efficient conversion efficiency. The output power instability is measured in the experiments with the value of less than 0.5% in four hours. The high efficient fiber lasers at 1018 nm with the stable output power can be applied to the tandem pumping high power fiber lasers as the excellent pump source.