Abstract: The radio frequency interference (RFI) is a dominant factor to degrade the image quality in microwave medical imaging systems. This paper analyzes the effects of RFI incurred in microwave medical imaging systems, and proposes a phase random coding method to transform RFI into zero mean random signal. The RFI can be suppressed easily using traditional mean value filter algorithm. A set of 2D breast cancer imaging experiments with simulated data demonstrate the proposed method is feasible and effective.