A narrow-linewidth single-mode distributed Bragg reflection ;DBR fiber laser is discussed.Through theoretical analysis,the single longitudinal mode operation domain is obtained.Then according to this theoretical result,a single-mode DBR fiber laser is fabricated.This laser is pumped by a semiconductor laser at 975.50 nm and the operating wavelength is 1556.91 nm.Its output power reaches 1.43 mW and the lasing linewidth is less than 1.2 MHz;instrument resolution limited .The measurement shows that the laser operates at single-mode.