Based on diode laser modulation technique, a 978 nm nanosecond-pulsed Yb-doped fiber laser is designed. The structure of the laser is master oscillator power amplifier. The laser system consists of the seed of modulated laser diode and one stage Yb-doped fiber amplifier with single-mode and single-cladding. The spectral central wavelength of laser diode seed is set as 978.3 nm by self-seeding. The laser pulse width after modulation is 4.5 ns and the repetition frequency is tunable at 10~50 MHz. The seed light is amplified to 115 mW by one stage Yb-doped fiber amplifier with single-cladding when the repetition frequency of laser diode seed modulation is 50 MHz. The corresponding central wavelength of the laser is 978.3 nm, 3 dB spectral bandwidth is 0.11 nm. The obvious amplified spontaneous emission does not appear in the spectrum after amplification.