We first present the parametric model of Radio Frequency Interference (RFI) suppression in Ultra-WideBand Synthetic Aperture Radar (UWB-SAR). Starting from the problem of signal parameter estimation, we deduce in detail the computation formula of RELAX algorithm for real-valued signal. As to the complexity of RFI, a Gradual RELAX (GRELAX) algorithm is proposed. Experiments with synthesized and raw RFI data demonstrate the efficiency of GRELAX in RFI suppression and its high processing speed.