Single-frequency fiber laser has the merits of excellent monochromaticity and high spectral power density, and it has widespread application needs in, for example, coherent communication and sensing, lidar, gravitational wave detection and nonlinear frequency conversion. At present, single-frequency fiber laser technologies are moving towards the direction of higher output power, broader spectral range, and higher overall performance, and have become the research frontier and hotspot in the field of laser technology. This manuscript systematically combed through the important progress of single-frequency fiber lasers in recent years. Specifically, the landmark works with respect to the implementation mode of single-frequency lasing, power scaling, wavelength expanding and performance improvement were reviewed. In addition, the current challenges as well as future trends of single-frequency fiber laser technology were also discussed.