Due to the fact that the near single frequency fiber laser is characterized with its unique advantages in coherent synthesis, optical communication, laser measurement and spectral analysis, the use of ring cavity fiber laser to achieve near single frequency output has become one of the research focuses for current researchers. Tunable fiber grating filters, tunable bandpass filters, tunable Fabry Perot (F-P) filters, and saturable absorbers are mainly used in ring cavity fiber lasers for a better near single frequency output, with the spectrum of the output laser affected by the filter bandwidth and the number of ring cavities. Therefore, based on fiber Bragg grating frequency selection and simulation, and by using Optisystem software, a near-single frequency laser output with a side mode suppression ratio of 88.20 dBm can be obtained by changing the number of ring cavities and the 3 dB bandwidth of the fiber grating.