Based on a master oscillator power amplifier (MOPA) design, the narrow linewidth fiber laser is a single frequency laser which has seed laser of MHz level. Optimally designed LiNbO3 phase modulator allows spectral broadening, the linewidth of output laser is at GHz level, it is possible to modulate the linewidth by means of radio frequency (RF) signal. The low noise fiber amplifier is used to extend the output power. This fiber laser is suitable for high power narrow linewidth single frequency amplifier system, the linewidth of GHz level enables SBS threshold to be greatly increased and provides output power at the kilowatt level, applicable to coherent beam combination (CBC) and spectral beam combination (SBC) system. The core technical parameters are displayed on LCD panel and the output power is adjustable, the fiber laser is widely applied in laboratory applications for its easy operation.