Abstract: Based on the principle of polarization modulation and optical interference, a microwave photonic filter with positive coefficients and negative coefficients is proposed and an experimental model is set up to verify the feasibility of the project design. With polarization modulation and the polarization property of light, it is achieved that the carrier and the first order sidebands are in two orthogonal polarization directions. Then, the phase shift is introduced to the carrier and the first order sidebands by adjusting the bias voltage of the polarization modulator. With the optical interference effect between the two orthogonal polarization lights in the fast and slow axes of the polarization maintaining fiber when they are in the same polarization state, a microwave photonic filter is achieved with negative coefficients or positive coefficients when the phase difference is 0 or 90. Finally, the frequency response of the filter with a frequency range between 0 GHz and 15 GHz is measured and verified.