Narrow linewidth fiber lasers can be used in applications such as remote sensing, nonlinear frequency conversion and beam combination. In this paper, the typical structures and corresponding working principles of narrow linewidth fiber laser oscillator were introduced. The single-frequency mode selecting technologies were summarized from the perspective of obtaining narrower linewidth, and the power scaling technologies were summarized from the perspective of generating higher power. The current status of narrow linewidth fiber laser oscillator was reviewed, and the development of narrow linewidth fiber laser technologies was prospected.