By modulating the laser diode, a nanosecond laser with center wavelength of 1064.14 nm, spectral width of 0.63 nm, repetition frequency of 1.55 MHz and pulse width of 11 ns is realized. This seed laser is then amplified to a high-power, high-repetition rate nanosecond fiber laser with an average output power of 18.2 W using a tri-stage master oscillator power amplifier (MOPA) configuration. This nanosecond fiber laser is used to pump a 145 m long tapered photonic crystal fiber with a self-made mode field adaptor (MFA), and a 2.2 W all-fiber white light supercontinuum source is realized with a 15 dB spectral flatness in the detected spectral range from 440 nm to 1700 nm. The spectral edge also corresponds well with the theoretical value.