This paper introduces an ultra-narrow linewidth Brillouin fiber laser based on distributed feedback fiber laser. By constructing a Brillouin ring cavity structure, a distributed feedback fiber laser is used as the Brillouin pump, a 980 nm semiconductor laser is used as the pump source for the intra-cavity erbium-doped laser amplification, thereby the Brillouin laser output with ultra-narrow linewidth is achieved. The Brillouin ring cavity is about 10 meters long, and a single longitudinal mode Brillouin laser is obtained with a proper polarization control. To ensure the single longitudinal mode laser output, the power of the distributed feedback fiber laser and the 980 nm pump source should be no less than 20 mW and 50 mW, respectively. With the output power of the pump source up to 250 mW, the final Brillouin laser output power exceeds 15 mW, and the linewidth is estimated to be less than 100 Hz.