The fiber laser linewidth is an important index reflecting the spatial resolution of laser radar and the measurement accuracy of laser precision measurement systems, and it has important application value in engineering practice. In this paper, based on the basic theory of linewidth measurement and the optical self-injection feedback method, the linewidth of the fiber laser before and after adding the optimized structure is compared, and the optimization effect of the optical self-injection dual-cavity feedback structure on the linewidth characteristics of the fiber laser is explored through experiments. A composite ring fiber laser and a self-injection dual-cavity feedback linewidth optimization structure are built, and the linewidth of the output light is measured by the delayed self-heterodyne method. The experimental results show that the optical self-injection dual-cavity feedback structure can further compress the fiber laser's average linewidth from 1.782 kHz to 1.319 kHz, and achieve a narrower linewidth laser output.