A fiber laser that can emit multi-wavelength simultaneously and each wavelength is single longitudinal mode is studied in this paper. The laser contains multiple laser composite resonators composed of optical fiber and fiber devices, each laser composite resonator uses a length of erbium-doped fiber as the gain medium and a fiber Bragg grating as the wavelength selector, so that each laser composite resonator can emit a specific wavelength at the same time, and the number of laser wavelengths emitted from the laser can be changed by changing the number of laser composite resonators. Each laser composite resonator is composed of two sub-resonators, by adjusting the cavity length difference between the two sub-resonators, the frequency interval of the composite resonator can be equivalent to the gain linewidth, so that the composite resonator will emit single longitudinal mode laser.