In order to make sure a microwave photonic filter both have wide tuning range and high frequency selectivity, a microwave photonic filter with a wide tuning range and narrow filter bandwidth based on a Brillouin oscillator is proposed and verified for the first time. The core of the filter is a Brillouin fiber oscillator with a cavity length of 10 m, and the stimulated Brillouin scattering pump and optical carrier signal are provided by two different tunable lasers. After the Brillouin gain spectrum interacts with the optical modulation sideband, the Brillouin fiber oscillator is used to narrow the spectral linewidth to realize narrowband microwave photonic filtering. By changing the pump wavelength, the filter passband can be tuned stably. The experimental results show that the microwave photonic filter can be stably tuned in the frequency range of 0-20 GHz. The out-of-band rejection ratio is found to be about 20 dB, and its 3-dB bandwidth and maximumQ value are 6.2 kHz and 3.222×10⁶, respectively.To the best of our knowledge, this is the highest value of a high-Q single-passband MPF reported to date. At the same time, the MPF has the advantages of wide tunability, high side mode suppression and a simple structure.