A tunable negative coefficient band-pass microwave photonic filter (MPF) with improved mainlobe-to-sidelobe suppression ratio (MSSR) by a Sagnac interferometer (SI) is experimentally demonstrated. The negative coefficient characteristic of MPF is achieved by using the phase modulation technology. The central frequency of the band-pass MPF is tunable continuously by changing the wavelength separation of multi-wavelength optical carriers. A 38-tap negative coefficient band-pass MPF whose MSSR is increased by about 6.371 dB is achieved in experiment by apodizing the tap coefficient with the Sagnac interferometer.