We report a photonic-assisted radio frequency self-interference cancellation (RF-SIC) and frequency down-conversion scheme based on a dual-polarization Mach-Zehnder modulator (DP-MZM) by using a polarization manipulation method. In the proposed scheme, the self-interference signal is directly canceled in the optical domain by simply controlling the polarization states between the output of the DP-MZM and the polarizer. Frequency down-conversion is achieved by optoelectronic frequency mixing. In the proof-of-concept experiment, the interference cancellation of the single-frequency signal and wideband signal was conducted. The proposed RF-SIC and frequency down-conversion system has flexible tunability, simple structure, and low cost, which can find potential applications in radio-over-fiber systems.