A tunable microwave photonic filter with variable coefficients including positive coefficients and negative coefficients is proposed,theoretic ally calculated and experimentally demonstrated using a polarization modulator.The polarizatio n modulator can conjugate phase modulation in the two orthogonal polarization di rections.After intensity modulation conversion,the initial microwave signal can be recovered through a photoelectric detector.The tunability of the filter tap coefficients between positive coefficients and negative coeffi cients is achieved only by adjusting the polarization controller to control the polarization coupling of th e optical signal,which results from the π phase change of the recovered microwave signal.A polarization beam splitter is used for the polarization coupling and re-combination of two optical signals for avoidin g i nterference.The tunability of the microwave photonic filter is achieved by changing its free spectral range (FSR) with adjusting the optical tunable time delay line in the light path to acquire different relative time delays.The freq uency responses of the microwave photonic filter with positive and negative coefficients under differen t relative time delays (300ps and 400ps) are measured,which coincide with the theoretical calculation and confirm the correctness of this scheme.