A novel narrow linewidth single longitudinal mode (SLM) Brillouin fiber laser (BFL) based on the feedback fiber loop (FFL) is proposed. The main single pass ring is constructed by an optical circulator and a 50 m single mode fiber (SMF) whose length is extended from 10 m of traditional BFL to 50 m used as laser gain medium. In order to guarantee the SLM operation, the FFL is configured with a 10 m SMF of traditional BFL. By theoretical analysis, threshold and linewidth of FFL-based BFL are better than those of traditional SLM BFL. In experiments, ring length is optimized by contrast with other two group different length SMFs. The system is enclosed in temperature control system so that environment disturbance is reduced. The SLM BFL with 99 mW threshold, 45 dB sidemode suppression and 0.8 dB power fluctuation in 1 h is obtained. 3.23 kHz linewidth is measured by delay interference method.