A high-power all-fiber Yb-doped picosecond fiber laser is achieved via a four-stage master oscillator power amplifier (MOPA) configuration. The seed source is a passively mode-locked fiber laser based on nonlinear polarization rotation (NPR) effect, with a central wavelength of 1062.8 nm, a repetition rate of 17.51 MHz, a spectral linewidth of 5 nm and an average power of 7.14 mW. To suppress the nonlinear effects in the process of power scaling, the repetition rate is upgrade to about 281.7 MHz through an all-fiber repetition rate extender. A 4.8 m large mode area (LMA) Yb-doped double clad fiber is utilized as gain medium in the main power amplifier. The maximal average output power of 31.2 W is obtained at the pump power of 60 W, corresponding to an optical-to-optical conversion efficiency of 52%. The output laser pulse has a central wavelength of 1063.7 nm, a pulse duration of 10.2 ps, a repetition rate of 281.7 MHz and a spectral linewidth of 7 nm. Moreover, the characteristics of the laser pulse are analyzed both in time and frequency domain.