A distributed feedback (DFB) fiber laser with a ratio of backward to forward output power of 100∶1 was composed by a 45mm length asymmetrical phase-shifted fiber grating fabricated on erbium-doped photosensitive fiber. Forward output laser was amplified and population inversion was got by using a certain length of Nufern EDFL980-Hp erbium-doped fiber to absorb surplus pump power after the active phase-shifted fiber grating. Using OptiSystem software, the best fiber length of the EDFL to get the highest gain was simulated. In order to keep the amplified laser with narrow line-width and low noise, a narrow-band light filter consisted of a fiber grating with the same Bragg wavelength as the laser and an optical circulator was used to filter the ASE noise of the out-cavity erbium-doped fiber. The designed laser structure sufficiently utilized the pump power, and a fiber laser of 32.5 mW output power, 11.5 kHz line width, and －87 dB/Hz relative intensity noise (RIN) at 300 mW of 980 nm pump power was gave.