For the first time, we propose a novel Brillouin random fiber laser with a narrow linewidth of ∼860 Hz based on the bi-directionally pumped stimulated Brillouin scattering (SBS) in a 10-km-long optical fiber. A random fiber Fabry-Perot (FP) resonator is built up through the pump depletion effects of SBS at both ends of the fiber. A theoretical model is developed to reveal the physics behind the unique random FP resonator that enables narrow random lasing emission. The novel laser is successfully applied for linewidth characterization beyond 860 Hz of light source under test.