Abstract: A simple high repitition-rate mode-locking fiber laser with narrow pulse width based on active modulation was designed and studied experimentally. A narrow linewidth continuous wave laser was modulated by a modulator with 4 GHz repetition-rate, and then, the pulse width can be narrowed to 2.6 ps from 27 ps by Raman gain soliton compression. The high repetition-rate mode-locking fiber laser was used to pump a 300 m long high nonlinear fiber, and the pulse width was extended to 7.4 ps. This supercontinuum (SC)has better flatness (a 250 nm SC spectrum can be observed at 20 dB) and higher stability (power fluctuation can be ±0.2 dB).