A novel high-speed frequency modulating scheme based on the self-injection locked single-frequency fiber laser is demonstrated. The laser frequency noise reaches -5 dB re Hz/Hz^1/2 at ~25 kHz, while the linewidth is estimated to be ~800 Hz. By modulating the length of external cavity to which the laser is locked, a stable frequency modulated laser with a modulation speed up to 160 kHz, and modulation amplitude >145 MHz at a modulation speed of 60 kHz are realized. The intensity noise spectrum of the frequency modulated fiber laser is found to be the same with that of the laser before modulation, except a series of gradually weakened harmonic peaks at the modulating frequency.