In order to achieve stable narrow line-width laser output,a narrow line-width erbium-doped fiber laser based on active fiber ring filter is designed,and its system theory and experimental analysis are discussed.The active fiber ring filter consists of a 50∶50 2×2 fused taper coupler and a 2 m erbium-doped fiber,and is capable of filtering out redundant longitudinal modes within the comb spectrum property of the system,so as to achieve the goal of narrowing the line-width.The experimental results show that,after incorporating the ring filter through fusion splicing,the side modes of the laser output are significantly suppressed.Specifically in the 0~400 MHz range,the laser works in a single longitudinal mode operation state,and the 3 dB line width of laser output is 3.4 kHz.