Based on home-made 100/400 double clad Yb-doped active silica fiber, a high power nanosecond Q-switched fiber laser with master oscillator power-amplifier (MOPA) structure and tunable repetition rate of 30-60 kHz is built. The center wavelength of system is 1064.08 nm. Using the 976 nm laser pumping, the output of average power of 761 W is achieved at the repetition rate of 60 kHz with a single pulse energy of 12.6 mJ, a slope efficiency of 83.4%, and a beam quality of Mx2=9.82, My2=9.02. At the repetition rate of 30 kHz, the average power is 526 W, the single pulse energy is 17.5 mJ, the slope efficiency is 85%, and the beam quality is Mx2=8.67, My2=8.57. This fiber is the reported domestic fiber with the highest average power and single pulse energy obtained at high repetition rate.