The key technologies of achieving single-frequency fiber lasers are how to enable the resonant cavities to have the high selectivity of modes, and how to properly use the narrow-width filters to help selecting modes. In addition, the fiber grating technology play an important role in the designing and optimizing fiber lasers. The main technologies and methods reported recently about the single or multiple wavelength single-frequency fiber lasers based on fiber gratings are summarized, their merits, demerits are analyzed and the developments are prospected.