We demonstrate a nanosecond fiber laser with tunable central wavelengths and narrow spectral bandwidths suitable for high-power amplification. Spectrally narrowband flat-top nanosecond pulses were generated at the fundamental repetition rate of 1.9 MHz in an Yb-doped fiber laser, which could be tuned in central wavelength from 1033 to 1053 nm by changing the nonlinear polarization rotation in the fiber laser cavity. In particular, such flat-top nanosecond pulses could be tuned around 1030 nm to match the gain bandwidth of ytterbium-doped double-clad fibers or 1053 nm to match the maximum gain in Nd-doped phosphate glass. The pulse duration could be changed from 1 to 15 ns by varying the pump power or laser polarization evolution in the cavity. By using an ytterbium-doped single-mode fiber preamplifier and a two-stage large-mode-area Yb-doped double-clad-fiber power amplifier, 280-W average power with pulse duration of 3 ns was obtained at 1034 nm.