We report an all-fiber-based master oscillator power amplifier picosecond ytterbium-doped fiber laser with an average power of 102 W and a spectral line width of 0.1 nm.The seed source is a compact single mode passively mode-locked fiber laser with an average power of 2.48 W.Finally,the laser maximum average output power of 102 W picosecond pulses is realized by a direct all-fiber amplifier structure in one stage.The experiment enables the optical-to-optical conversion efficiency to reach 61.4％,with the central wavelength of 1063.7nm.A significant feature of this experiment is the spectral line width of 0.1 nm.The spectrum has no broadening or nonlinear effects when the pump is strengthened.Fiber lasers have drawn a great deal of atteution due to their admirable beam quality,conversion efficiency,high stability and high heat dissipation.[1-3]Recently,high power fiber lasers have produced stronger power.With the rapid development of the fiber and supporting technology,especially that of cladding pump technology based on double-cladding fiber,[4] fiber laser power has been enhanced over the level of ten thousand watt.[5] Ultra-short pulsed fiber lasers at the level of picoseconds are being widely applied in the fields of industrial manufacture,medical treatment,telecommunication,military and so on[6]because they have high peak power and good thermal stability.At present,the average power output of an all-fiber master oscillator power amplifier (MOPA) pi eosecond laser svstem has reached more than 300 W.[6]