A photonic-assisted wideband frequency downconversion with simultaneous self-interference cancellation (SIC) and image rejection (IR) based on a dual-polarization Mach-Zehnder modulator (DPol-MZM) is proposed. The SIC is achieved by combining the DPol-MZM, a polarization controller, and a polarizer. The IR is performed by using a wavelength division multiplexer, two photodetectors, and an electrical 90-degree hybrid coupler. Because the received and assisted signals are injected into the system independently of each other, the system has strong robustness. The performance of the system is demonstrated through simulation experiments. The SIC depth higher than 35 dB and the image rejection ratio larger than 55 dB within 1 GHz bandwidth are achieved. Besides, the recovery 16-QAM signal has a clear constellation diagram and an EVM of 8.4%.